Fix VMS build broken by d1718a7cf5

Fix VMS build broken by d1718a7cf5

This comment in perl.c:

				/* Note: 20,40,80 used for NATIVE_HINTS */

(added by a0ed51b3 [Here are the long-expected Unicode/UTF-8 mod-
ifications.]), has apparently always been wrong.  The values in
vms/vmsish.h end with 7 zeroes in hex, and are only shifted down to
one zero when assigned to cop->op_private in op.c:newSTATEOP.  In
PL_hints they never have the values indicated in perl.h.  So those
are actually free bits.  It’s the high versions in vmsish.h that
are not free.

20 (actually 0x20000000) hasn’t been used for anything since commit
744a34f9085 (Urk -- undo previous removal of vmsish 'exit' change),
which change I don’t really understand.  In any case, the comment was
never updated.

This comment in op.h:

/* NOTE: OP_NEXTSTATE and OP_DBSTATE (i.e. COPs) carry lower
 * bits of PL_hints in op_private */

was added by d41ff1b8ad98 (introduce $^U), which was later reverted.
op_private does not carry the lower bits of PL_hints, but, rather,
certain higher bits of PL_hints, shifted to fit (the NATIVE_HINTS
cited above).

Due to misleading comments, I ended up breaking the VMS build in com-
mit d1718a7cf5, by using its bits for something else.

This commit moves the bits around a bit to avoid the clash, and modi-
fies the comments to reflect reality.