Sort /etc/groups, fixes #55.

Ensures that the same configurations will produce identical files.  The
order of the lines used to change and triggered spurious differences.
Sort groups by line number then by name.

Existing groups will keep their position in /etc/group, and new groups
will be appended at the end of the file in a reproduceable manner.
Merge pull request #56 from piojo/accounts-sorted

Sort /etc/groups, fixes #55.