(#22143) Add 'gid' fact to resolve users primary group.

A very simple fact to fetch the primary group.  I've only tested
this on Linux/OSX/FreeBSD but id -ng seems to be global.  I even
got a user to test it on Hurd and it worked.
Merge pull request #514 from apenney/gid

(#22143) Add 'gid' fact to resolve users primary group.