Added basic rfc8527 support, but without support for:
* ds:operational
* with-origin
* clixon/test/test_rfc8527_*.sh
The current level of rfc8527 support is intended to allow commit
operations on the 'candidate' datastore without rewriting the 'startup'
datastore.
* `201 Created` for created resources
* `204 No Content` for replaced resources.
* See [RESTCONF: HTTP return codes are not according to RFC 8040](https://github.com/clicon/clixon/issues/56)
* HTTP `Location:` fields added in RESTCONF POST replies