Merge pg_shadow && pg_group -- UNTESTED

Greetings,

Here's a proof-of-concept pretty much untested (it compiles) patch
against HEAD for review of the general approach I'm taking to
merging pg_shadow and pg_group. This is in order to support group
ownership and eventually roles. This patch includes my grammar and
get_grosysid move patches, and so conflicts with them.

I'm going to hold off on testing it very much in hopes that I'll get
some feedback on it. A good place to start commenting would probably
be my changes to pg_shadow.h.

Thanks,

Stephen