Extend the BKI infrastructure to allow system catalogs to be given

Extend the BKI infrastructure to allow system catalogs to be given
hand-assigned rowtype OIDs, even when they are not "bootstrapped" catalogs
that have handmade type rows in pg_type.h.  Give pg_database such an OID.
Restore the availability of C macros for the rowtype OIDs of the bootstrapped
catalogs.  (These macros are now in the individual catalogs' .h files,
though, not in pg_type.h.)

This commit doesn't do anything especially useful by itself, but it's
necessary infrastructure for reverting some ill-considered changes in
relcache.c.