* Yang 1.1 feature and if-feature according to RFC 7950 7.20.1 and 7.20.2.
* See https://github.com/clicon/clixon/issues/41
* Features are declared via CLICON_FEATURE in the configuration file.
* logical combination of features not implemented, eg if-feature "not foo or
* Identity without any identityref:s caused SEGV
* 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