pg_dump: use ALTER TABLE for PKs

I've attached a patch which enhances pg_dump to use ALTER TABLE when
defining the primary keys of a table (instead of including this
information in the table definition).

This follows a suggestion by Christopher Kings-Lynne (good idea!).

The patch is pretty simple, partly because the code was already there,
just commented out ;-) I fixed a typo and added some comments.

Bruce suggested that this should get some code review, since a bug in
pg_dump would be bad news. Any comments would be welcome.

Cheers,

Neil

--
Neil Conway <neilconway@rogers.com>
PGP Key ID: DB3C29FC