New `ca_system_only` backend callback for reading system-only data
New `CLICON_XMLDB_SYSTEM_ONLY_CONFIG` configuration option
API: Added `system_only` parameter to clixon_xml2file1()
Cleared running on commit and inited candidate on startup with system-only data
Added callback code in main example
A new extension added in clixon-lib.yang
A conditional in xmldb_dump to not write system-only data to datastores
A test for verifying system-only data is not written at edit-commit
