pgsql: Put a CHECK_FOR_INTERRUPTS call into the loops that try to find a

Log Message:
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Put a CHECK_FOR_INTERRUPTS call into the loops that try to find a unique new
OID or new relfilenode. If the existing OIDs are sufficiently densely
populated, this could take a long time (perhaps even be an infinite loop),
so it seems wise to allow the system to respond to a cancel interrupt here.
Per a gripe from Jacky Leng.

Backpatch as far as 8.1. Older versions just fail on OID collision,
instead of looping.

Tags:
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REL8_1_STABLE

Modified Files:
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pgsql/src/backend/catalog:
catalog.c (r1.64 -> r1.64.2.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/catalog.c?r1=1.64&r2=1.64.2.1)