Make oidin/oidout produce and consume unsigned representation of Oid,

rather than just being aliases for int4in/int4out.  Give type Oid a
full set of comparison operators that do proper unsigned comparison,
instead of reusing the int4 comparators.  Since pg_dump is now doing
unsigned comparisons of OIDs, it is now *necessary* that we play by
the rules here.  In fact, given that btoidcmp() has been doing unsigned
comparison for quite some time, it seems likely that we have index-
corruption problems in 7.0 and before once the Oid counter goes past
2G.  Fixing these operators is a necessary step before we can think
about 8-byte Oid, too.