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
Improved diff algorithm for XML and TEXT/curly, replaced UNIX diff with structural in-mem algorithm
Fixed: ["show compare" and "show compare | display cli" differs #23](https://github.com/clicon/clixon-controller/issues/23)
Changed parameters of example clispec function `compare_dbs()`
Added show2cbuf functions for TEXT/CLI
Renamed clixon_txt2file to clixon_text2file
New API is as follows:
* `clixon_xml2file()` - Print internal tree as XML to file
* `clixon_xml2cbuf()` - Print internal tree as XML to buffer
* `clixon_json2file()` - Print internal tree as JSON to file
* `clixon_json2cbuf()` - Print internal tree as JSON to buffer
* `clixon_cli2file()` - Print internal tree as CLI format to file
* `clixon_txt2file()` - Print internal tree as text format to file
* Added `skiptop` parameter, if set only apply to children of a node, skip top node
* default is 0
* Functions are merged, ie removed and replaced with more generic functions
* `xml2json_cbuf()`: Added `skiptop` parameter: `xml2json_cbuf(..., int skiptop)`
* `xml2json()` and `xml2json_cb()` merged into `xml2json_file()` with `skiptop`
* Replace `xml2json(...)` with `xml2json_file(..., stdout, 0)`
* Replace `xml2json_cb(...)` with `xml2json_file(..., 0)`
* `clicon_xml2cbuf()`: Added `skiptop` parameter: `clicon_xml2cbuf(..., int skiptop)`
* `xml2cli()`: Added `skiptop` parameter: `xml2cli(..., int skiptop)`
* `clixon_xml_parse_string()`
* `clixon_xml_parse_file()`
* New JSON parsing API, with same signature as XML parsing:
* `clixon_json_parse_string()`
* `clixon_xml_parse_file()`
* XML YANG binding API have been rearranged as follows:
* `xml_bind_yang_rpc()`
* `xml_bind_yang_rpc_reply()`
* `xml_bind_yang()`
* `xml_bind_yang0()`
