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

contrib/postgres_fdw/postgres_fdw.control

@@ -0,0 +1,5 @@
+# postgres_fdw extension
+comment = 'foreign-data wrapper for remote PostgreSQL servers'
+default_version = '1.0'
+module_pathname = '$libdir/postgres_fdw'
+relocatable = true