postgres_fdw: Consider foreign joining and foreign sorting together.

Commit ccd8f979 gave us the ability to
request that the remote side sort the data, and, later, commit
e4106b25 gave us the ability to
request that the remote side perform the join for us rather than doing
it locally.  But we could not do both things at the same time: a
remote SQL query that had an ORDER BY clause would never be a join.
This commit adds that capability.

Ashutosh Bapat, reviewed by me.