This patch implements putting language handlers for the optional PLs

This patch implements putting language handlers for the optional PLs
into pg_catalog rather than public, and supports dumping languages whose
handlers are found there. This will make it easier to drop the public
schema if desired.

Unlike the previous patch, the comments have been updated and I have
reformatted some code to meet Alvarro's request to stick to 80 cols. (I
actually aghree with this - it makes printing the code much nicer).

I think I did the right thing w.r.t versions earlier than 7.3, but I
have no real way of checking, so that should be checked by someone with
more/older knowledge than me ;-)

Andrew Dunstan