pgsql: Handle OID column inheritance correctly in ALTER TABLE ... INHER

Handle OID column inheritance correctly in ALTER TABLE ... INHERIT.

Inheritance operations must treat the OID column, if any, much like
regular user columns. But MergeAttributesIntoExisting() neglected to
do that, leading to weird results after a table with OIDs is associated
to a parent with OIDs via ALTER TABLE ... INHERIT.

Report and patch by Amit Langote, reviewed by Ashutosh Bapat, some
adjustments by me. It's been broken all along, so back-patch to
all supported branches.

Discussion: https://postgr.es/m/cb13cfe7-a48c-5720-c383-bb843ab28298@lab.ntt.co.jp

Branch
------
REL9_3_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/5f89a9885e118ab22689f9641237f7941d9ba8fd

Modified Files
--------------
src/backend/commands/tablecmds.c | 33 +++++++++++++++++++++++
src/test/regress/expected/inherit.out | 49 +++++++++++++++++++++++++++++++++++
src/test/regress/sql/inherit.sql | 26 +++++++++++++++++++
3 files changed, 108 insertions(+)