Redmine#3283: implement ^meta protocol extension and getvariablemetatags+getclassmetatags functions

This is finishing up the work defined in https://cfengine.com/dev/issues/3283

* implement `getvariablemetatags(varname)` function to get the `meta` tags of a variable (with acceptance test)
* implement `getclassmetatags(varname)` function to get the `meta` tags of a class (with acceptance test)
* implement `^meta=a,b,c` extension to module protocol (acceptance test `tests/acceptance/08_commands/01_modules/set-tags.cf` is **not** working yet, review welcome)