Missing importing option of postgres_fdw

Hi,

I noticed that there is no postgres_fdw option to control whether check
constraints on remote tables are included in the definitions of foreign
tables imported from a remote PG server when performing IMPORT FOREIGN
SCHEMA, while we now allow check constraints on foreign tables.

Attached is a patch for that.  I'll add this to the next CF.

Best regards,
Etsuro Fujita