Allow queries submitted by postgres_fdw to be canceled.

This fixes a problem which is not new, but with the advent of direct
foreign table modification in 0bf3ae88,
it's somewhat more likely to be annoying than previously.  So,
arrange for a local query cancelation to propagate to the remote side.

Michael Paquier, reviewed by Etsuro Fujita.	 Original report by
Thom Brown.