Fix gidNumber conflicts for service groups vs. project groups.

Previously I was trying to be clever and allow different projects
to use the same gids for service groups.  My logic wasn't
clever enough, though, and service groups were sometimes
trying to grab gids that were already allocated for project groups.

So... now the code is dumber but should work better.  We just
use a monotonically increasing gid for either service or project
groups, regardless of project scope.

Change-Id: I94019cb2c494ef7fe223c520c7d249de370fb886