pgsql: Fix plpgsql's issues with dropped columns in rowtypes in 8.4 bra

Fix plpgsql's issues with dropped columns in rowtypes in 8.4 branch.

This is a back-patch of commit dcb2bda9b7042dbf43f876c94ebf35d951de10e9 of
Aug 6 2009, which fixed assorted cases in which plpgsql would fail to cope
with composite types that contain any dropped columns.  Per discussion,
this fix has been out in 9.0 for long enough to make it improbable that it
creates any new bugs, so this is a low-risk fix.  To make it even lower
risk, I did not back-patch the changes in execQual.c, but just accepted
the duplication of code between there and tupconvert.c.  The added files
tupconvert.h and tupconvert.c match their current states in HEAD.

Branch
------
REL8_4_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/5d3853a7fa40b28b44b14084863fd83a188c9a9e

Modified Files
--------------
src/backend/access/common/Makefile     |    3 +-
src/backend/access/common/tupconvert.c |  365 ++++++++++++++++++++++++++++++++
src/include/access/tupconvert.h        |   44 ++++
src/pl/plpgsql/src/nls.mk              |    2 +-
src/pl/plpgsql/src/pl_exec.c           |   99 ++++-----
src/test/regress/expected/plpgsql.out  |   51 +++++
src/test/regress/sql/plpgsql.sql       |   30 +++
7 files changed, 538 insertions(+), 56 deletions(-)