* 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