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
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REL9_2_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/6c4cf2be81e4b783402aecf49df2f1120e42b99b

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(+)