w-level general upgrades and as a complement to module-specific upgrade.
* Called on startup after initial XML parsing, but before module-specific upgrades
* Enabled by definign the `.ca_datastore_upgrade`
* [General-purpose upgrade documentation](https://clixon-docs.readthedocs.io/en/latest/backend.html#ge
neral-purpose)
* JSON parse error messages change from ` on line x: syntax error,..` to `json_parse: line x: syntax err
or`
* Unknown-element error message is more descriptive, eg from `namespace is: urn:example:clixon` to: `Fai
led to find YANG spec of XML node: x with parent: xp in namespace urn:example:clixon`.
* C-API parse and validation API more capable
* `xml_spec_populate` family of functions extended with three-value return values
* -1: error, 0: parse OK, 1: parse and YANG binding OK.
* `xml_parse` and `json_parse` API changes
* Three value returns: -1: error, 0: parse OK, 1: parse and YANG binding OK.
* Extended `xml_parse_file2` and `xml_parse_string2` extended API functions with all options available.
* New concept called `yang_bind` that defines how XML symbols are bound to YANG after parsing
* Existing API same except `xml_parse_file` `endtag` argument moved to `xml_parse_file2`
* C-API: Added instrumentation: `xml_size` and `xml_stats_get`.
* Fixed: Enabling modstate (CLICON_XMLDB_MODSTATE), changing a revision on a yang, and restarting made the backend daemon exit at start (thanks Matt)
* Also: ensure to load `ietf-yang-library.yang ` if CLICON_XMLDB_MODSTATE is set
* Restconf RFC 8040 increased feature compliance
* Cache-Control: no-cache added in HTTP responses (RFC Section 5.5)
* Restconf monitoring capabilities (RFC Section 9.1)
* Added support for Yang extensions
* New plugin callback: ca_extension
* Main backend example includes example code on how to implement a Yang extension in a plugin.
* JSON changes
* Non-pretty-print output removed all extra spaces.
* Example: `{"nacm-example:x": 42}` --> {"nacm-example:x":42}`
* Empty JSON container changed from `null` to `{}`.
* Empty list and leafs remain as `null`
* Removed unnecessary configure dependencies
* libnsl, libcrypt, libm, if_vlan,...
* pseudo-plugin added, to enable callbacks also for main programs. Useful for extensions
* Yang Unique statements with multiple schema identifiers did not work on some platforms due to memory error.
* Accepted:
```
curl -X PUT http://localhost/restconf/data/mod:a -d {"mod:a":"x"}
```
* Not accepted (must prefix "a" with module):
```
curl -X PUT http://localhost/restconf/data/mod:a -d {"a":"x"}
```
* Undefine `RESTCONF_NS_DATA_CHECK` in include/clixon_custom.h to disable strict check.
* clixon-config YAML file has new revision: 2019-03-05.
* New URN and changed top-level symbol to `clixon-config`
* Removed obsolete `_CLICON_XML_NS_STRICT` variable and `CLICON_XML_NS_STRICT` config option.
* Added clicon_socket_set() and clicon_socket_get() functions for cleaning up backend server and restconf FCGI socket on termination.
* Removed obsolete `-x` command-line option to clixon_cli.
* Moved datastore/text/ code to datastore/ since there is only one type of datastore.
* Added "magic script line" to test script for sourced and scriped exits
* Added -p <dir> command-line option to all programs: backend, cli, netconf, restconf.
* Moved and updated all standard ietf and iana yang files from example and yang/ to `yang/standard`.
* Renamed example yang from example.yang -> clixon-example.yang
* For backward compatibility, load of startup and running set CLICON_XML_NS_STRICT to false temporarily.
* Added three-valued return values for several validate functions where -1 is fatal error, 0 is validation failed and 1 is validation OK.
* This includes: `xmldb_put`, `xml_yang_validate_all`, `xml_yang_validate_add`, `xml_yang_validate_rpc`, `api_path2xml`, `api_path2xpath`
* Added new xml functions for specific types: `xml_child_nr_notype`, `xml_child_nr_notype`, `xml_child_i_type`, `xml_find_type`.
* For Example, adding bad-, missing-, or unknown-element error messages, etc instead of operation-failed
* Removed delete-config support for candidate db since it is not supported in RFC6241.
* Switched the order of `error-type` and `error-tag` in all netconf and restconf error messages to comply to RFC order.
* Added example_rpc RPC to example backend
* Renamed xml_namespace[_set]() to xml_prefix[_set]()
* Some restconf error messages contained "rpc-reply" or "rpc-error" which have now been removed.
* Netconf/Restconf RPC extra input arguments are ignored (https://github.com/clicon/clixon/issues/47)
* NACM module support (RFC8341 A1+A2)
* Recovery user "_nacm_recovery" added.
* Example use is restconf PUT when NACM edit-config is permitted, then automatic commit and discard are permitted using recovery user.
* Example user changed adm1 to andy to comply with RFC8341 example
* Yang code upgrade (RFC7950)
* RPC method input parameters validated
* see https://github.com/clicon/clixon/issues/4
* Correct XML namespace handling
* XML multiple modules was based on "loose" semantics so that yang modules were found by iterating thorugh namespaces until a match was made. This did not adhere to proper [XML namespace handling](https://www.w3.org/TR/2009/REC-xml-names-20091208), and causes problems with overlapping names and false positives. Below see XML accepted (but wrong), and correct namespace declaration:
```
<rpc><my-own-method></rpc> # Wrong but accepted
<rpc xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"> # Correct
<my-own-method xmlns="http://example.net/me/my-own/1.0">
</rpc>
```
* To keep old loose semantics set config option CLICON_XML_NS_ITERATE (true by default)
* XML to JSON translator support for mapping xmlns attribute to module name prefix.
* Default namespace is still "urn:ietf:params:xml:ns:netconf:base:1.0"
* See https://github.com/clicon/clixon/issues/49
* Changed all make tags --> make TAGS
* Keyvalue datastore removed (it has been disabled since 3.3.3)
* debug rpc added in example application (should be in clixon-config).
* Improved unknown handling
* Configure option `CLICON_YANG_DIR` is changed from a single directory to a path of directories
* Note CLIXON_DATADIR (=/usr/local/share/clixon) need to be in the list
* Clarified semantics of main yang module:
* -y option to commands MUST specify filename
* CLICON_YANG_MODULE_MAIN MUST specify a module
* yang_parse() changed to take either filename or module name and revision.
* Removed clicon_dbspec_name[_set]().
* Use yang_main_module_name() instead.
* Replaced yang_spec_main with yang_spec_parse_module
* Added yang_spec_parse_file
* That is, limited support of: ietf-yang-library.yang
* For example: `<module><name>example</name><revision/></module><module><name>ietf-restconf-monitoring</name><revision>2017-01-26</revision></module>...`
* Comply to RFC 8040 3.5.3.1 rule: api-identifier = [module-name ":"] identifier
* The "module-name" was a no-op before.
* This means that there was no difference between eg: GET /restconf/data/ietf-yang-library:modules-state and GET /restconf/data/XXXX:modules-state
* Previous support did no validation of values.
* Validation of types and CLI expansion
* Example extended with inclusion of iana-if-type RFC 7224 interface identities
