pgsql: Don't require a user mapping for FDWs to work.

Don't require a user mapping for FDWs to work.

Commit fbe5a3fb73102c2cfec11aaaa4a67943f4474383 accidentally changed
this behavior; put things back the way they were, and add some
regression tests.

Report by Andres Freund; patch by Ashutosh Bapat, with a bit of
kibitzing by me.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/5d4171d1c70edfe3e9be1de9e66603af28e3afe1

Modified Files
--------------
contrib/file_fdw/input/file_fdw.source | 3 ++
contrib/file_fdw/output/file_fdw.source | 14 +++++-
contrib/postgres_fdw/expected/postgres_fdw.out | 61 +++++++++++++++++++++++---
contrib/postgres_fdw/postgres_fdw.c | 10 +++++
contrib/postgres_fdw/sql/postgres_fdw.sql | 9 ++--
src/backend/foreign/foreign.c | 36 +++++++++++----
src/backend/optimizer/util/relnode.c | 12 ++++-
src/include/foreign/foreign.h | 2 +-
8 files changed, 125 insertions(+), 22 deletions(-)