pgsql: Make ALTER TABLE revalidate uniqueness and exclusion constraints

Make ALTER TABLE revalidate uniqueness and exclusion constraints.

Failure to do so can lead to constraint violations.  This was broken by
commit 1ddc2703a936d03953657f43345460b9242bbed1 on 2010-02-07, so
back-patch to 9.0.

Noah Misch.  Regression test by me.

Branch
------
REL9_0_STABLE

Details
-------
http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=39b5e5f3370258cae843e8cc83eccd59ddb532dd

Modified Files
--------------
src/backend/catalog/index.c               |   43 ++++++++++++++++-------------
src/backend/commands/cluster.c            |    9 +++++-
src/backend/commands/indexcmds.c          |    4 +-
src/backend/commands/tablecmds.c          |    7 +++--
src/include/catalog/index.h               |    5 +++-
src/include/commands/cluster.h            |    1 +
src/test/regress/expected/alter_table.out |    4 +++
src/test/regress/sql/alter_table.sql      |    2 +
8 files changed, 48 insertions(+), 27 deletions(-)