Adjust pg_cast.h so that the OIDs assigned to built-in casts come from

Adjust pg_cast.h so that the OIDs assigned to built-in casts come from
genbki.sh's pool (10000-16383) instead of being run-time assigned by
heap_insert.  Might as well use the pool as long as it's there ...
I was a bit bemused to realize that it hadn't been in use at all since 7.2.

initdb not forced since this doesn't really affect anything.  The OIDs
of casts and system indexes will change next time you do one, though.