This include plugin module init and exit functions
New type: clixon_plugin_t exposed via public API while struct clixon_plugin is private
- Makefile changes for static linkage using "LINKAGE=static"
- Moved nacm external init to later stage in backend_main
* Added new revision of main example yang: `clixon-example@2020-12-01.yang`
* Fixed [YANG: key statement in rpc/notification list #148](https://github.com/clicon/clixon/issues/148)
* Do not check uniqueness among lists without keys
* If fails, exit with error message, eg: `Yang error: Sanity check failed: LIST vsDataContainer lacks key statement which MUST be present (See RFC 7950 Sec 7.8.2)`
* Can be disabled by setting `CLICON_CLICON_YANG_LIST_CHECK` to `false`
* Fixed multi-namespace for augmented state which was not covered in 4.2.0.
* The multi-namespace augment state may rearrange the XML namespace attributes.
* Mandatory variables can no longer be deleted.
* 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.
