src/libpcp/context.c: mostly cosmetic changes
src/libpcp/context.c: mostly cosmetic changes

Add some diagnostic CHECK macros.

Rename a parameter to pmGetContextHostName() and
pmGetContextHostName_r() to try and reduce "context handle vs context
index slot" confusion.

Add comments to explain some stuff and mark some stuff as TODO.

Start to retire __pmCurrentContext() which is a relatively recent
bad idea.

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