Fix contrib/pg_trgm to have smoother updates from 9.0.

Take care of some loose ends in the update-from-unpackaged script, and
apply some ugly hacks to ensure that it produces the same catalog state
as the fresh-install script.  Per discussion, this seems like a safer
plan than having two different catalog states that both call themselves
"pg_trgm 1.0", even if it's not immediately clear that the subtle
differences would ever matter.

Also, fix the stub function gin_extract_trgm() so that it works instead
of just bleating.  Needed because this function will get called during a
regular dump and reload, if there are any indexes using its opclass.
The user won't have an opportunity to update the extension till later,
so telling him to do so is unhelpful.