* 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
* CLICON_NACM_MODE config option, default is disabled.
* Added username attribute to all rpc:s from frontend to backend
* Added NACM backend module in example
* Builds and installs a new restconf library: libclixon_restconf.so and clixon_restconf.h
* The restconf library can be included by a restconf plugin.
* Example code in example/Makefile.in and example/restconf_lib.c
* Authorization
* Example extended with authorization
* Test added with http basic authorization (test/test_auth.sh)
* Documentation in FAQ.md
* README.md extended with new yang, netconf, restconf, datastore, and auth sections.
