Allow a foreign table CHECK constraint to be initially NOT VALID.

Allow a foreign table CHECK constraint to be initially NOT VALID.

For a table, the constraint can be considered validated immediately,
because the table must be empty.  But for a foreign table this is
not necessarily the case.

Fixes a bug in commit f27a6b15e6566fba7748d0d9a3fc5bcfd52c4a1b.

Amit Langote, with some changes by me.

Discussion: http://postgr.es/m/d2b7419f-4a71-cf86-cc99-bfd0f359a1ea@lab.ntt.co.jp