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