pgsql: Fix inheritance count tracking in ALTER TABLE ..

Log Message:
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Fix inheritance count tracking in ALTER TABLE .. ADD CONSTRAINT.

Without this patch, constraints inherited by children of a parent
table which itself has multiple inheritance parents can end up with
the wrong coninhcount. After dropping the constraint, the children
end up with a leftover copy of the constraint that is not dumped
and cannot be dropped. There is a similar problem with ALTER TABLE
.. ADD COLUMN, but that looks significantly more difficult to
resolve, so I'm committing this fix separately.

Back-patch to 8.4, which is the first release that has coninhcount.

Report by Hank Enting.

Tags:
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REL8_4_STABLE

Modified Files:
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pgsql/src/backend/commands:
tablecmds.c (r1.288.2.5 -> r1.288.2.6)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c?r1=1.288.2.5&r2=1.288.2.6)