For REASSIGN OWNED for foreign user mappings

As reported in bug #13809 by Alexander Ashurkov, the code for REASSIGN
OWNED hadn't gotten word about user mappings.  Deal with them in the
same way default ACLs do, which is to ignore them altogether; they are
handled just fine by DROP OWNED.  The other foreign object cases are
already handled correctly by both commands.

Also add a REASSIGN OWNED statement to foreign_data test to exercise the
foreign data objects.  (The changes are just before the "cleanup" phase,
so it shouldn't remove any existing live test.)

Reported by Alexander Ashurkov, then independently by Jaime Casanova.