pgsql: Revert misguided change to postgres_fdw FOR UPDATE/SHARE code.

Revert misguided change to postgres_fdw FOR UPDATE/SHARE code.

In commit 462bd95705a0c23ba0b0ba60a78d32566a0384c1, I changed postgres_fdw
to rely on get_plan_rowmark() instead of get_parse_rowmark().  I still
think that's a good idea in the long run, but as Etsuro Fujita pointed out,
it doesn't work today because planner.c forces PlanRowMarks to have
markType = ROW_MARK_COPY for all foreign tables.  There's no urgent reason
to change this in the back branches, so let's just revert that part of
yesterday's commit rather than trying to design a better solution under
time pressure.

Also, add a regression test case showing what postgres_fdw does with FOR
UPDATE/SHARE.  I'd blithely assumed there was one already, else I'd have
realized yesterday that this code didn't work.

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/8ec8760fc87ecde0516e511f1c55aec627b01ea7

Modified Files
--------------
contrib/postgres_fdw/expected/postgres_fdw.out |   33 ++++++++++++++++++++++++
contrib/postgres_fdw/postgres_fdw.c            |   17 +++++-------
contrib/postgres_fdw/sql/postgres_fdw.sql      |    5 ++++
3 files changed, 45 insertions(+), 10 deletions(-)