Redmine#3283: implement ^tags protocol extension and getvariabletags+getclasstags functions

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