pgsql: postgres_fdw: Refactor deparsing code for locking clauses.

postgres_fdw: Refactor deparsing code for locking clauses.

The upcoming patch to allow join pushdown in postgres_fdw needs to use
this code multiple times, which requires moving it to deparse.c. That
seems like a good idea anyway, so do that now both on general principle
and to simplify the future patch.

Inspired by a patch by Shigeru Hanada and Ashutosh Bapat, but I did
it a little differently than what that patch did.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/b88ef201d46e6519b5e0589358c952a4c0f5bf0f

Modified Files
--------------
contrib/postgres_fdw/deparse.c | 60 +++++++++++++++++++++++++++++++++++++
contrib/postgres_fdw/postgres_fdw.c | 51 ++-----------------------------
contrib/postgres_fdw/postgres_fdw.h | 2 ++
3 files changed, 64 insertions(+), 49 deletions(-)