postgres_fdw: Teach IMPORT FOREIGN SCHEMA about partitioning.

Don't import partitions.  Do import partitioned tables which are
not themselves partitions.

Report by Stephen Frost.  Design and patch by Michael Paquier,
reviewed by Amit Langote.  Documentation revised by me.

Discussion: http://postgr.es/m/20170309141531.GD9812@tamriel.snowman.net