Proposed patch: make pg_dump --data-only consider FK constraints

Okay, I got tired of seeing people complain about foreign-key constraint
violations in data-only dumps. While it's true that the problem can't
be solved in the general case (because of potentially circular
references), we could certainly make pg_dump at least *try* to order the
tables according to foreign key relationships. It turns out not to even
require a whole lot of new code. Accordingly I propose the attached
patch. It will order the tables safely if it can, and otherwise
complain like this:

pg_dump: WARNING: circular foreign-key constraints among these table(s):
pg_dump: master
pg_dump: child
pg_dump: You may not be able to restore the dump without using --disable-triggers or temporarily dropping the constraints.

Comments?

regards, tom lane