Add postgres_fdw contrib module.

There's still a lot of room for improvement, but it basically works,
and we need this to be present before we can do anything much with the
writable-foreign-tables patch.  So let's commit it and get on with testing.

Shigeru Hanada, reviewed by KaiGai Kohei and Tom Lane
Teach MSVC build system about postgres_fdw.

Per buildfarm.
Need to decorate XactIsoLevel as PGDLLIMPORT for postgres_fdw.

Per buildfarm.
Remove useless variable

Per Jeff Janes
Move relpath() to libpgcommon

This enables non-backend code, such as pg_xlogdump, to use it easily.
The previous location, in src/backend/catalog/catalog.c, made that
essentially impossible because that file depends on many backend-only
facilities; so this needs to live separately.