Put a CHECK_FOR_INTERRUPTS call into the loops that try to find a unique new

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.