python: update pmapi.py labelset functions to be more pythonic
python: update pmapi.py labelset functions to be more pythonic

Update the label/labelset related API functions to be more
pythonic by returning dicts where appropriate rather than lists
or arrays of pmLabelSet C structures.

Also add more helper pmapi bindings, e.g. pmID_build() and
similar functions.

Documentation still TODO. QA/1310 exercises this.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp