Re: Splitting up guc.c

Here's a v3 that gets rid of guc_hooks.c in favor of moving the
hook functions to related modules (though some did end up in
variables.c for lack of a better idea).  I also pushed all the
hook function declarations to guc_hooks.h.  Unsurprisingly,
removal of guc.h #includes from header files led to discovery
of some surprising indirect dependencies, notably a lot of places
were evidently depending on indirect inclusions of array.h.

I think this is code-complete at this point.  I'd like to not
sit on it too long, because it'll inevitably get side-swiped
by additions of new GUCs.  On the other hand, pushing it in
the middle of a CF would presumably break other people's patches.
Maybe push it at the end of this CF, to give people a month to
rebase anything that's affected?

			regards, tom lane