Status:  Include more information about groups

This reworks slightly how the group names are sent down the pipe.  Rather
than iterating over the sc->group_names array, instead go through all groups
and pull out not only their name, but their shortname (the group number,
basically), as well as the number of clients on each group.

This will then allow writers of status output to decide whether they wish to
show all groups, or just groups with clients on, etc.
read_status: rework how groups are displayed

The group list now no longer shows all groups by default.  Instead, those
groups without any clients are hidden.