UCB_BUFOUT, moving i/o buffers back into kernel space

[utzoo!decvax!ucbvax!virus!mike]

Depending on your devices and configuration, it is not always necessary
to do spl7's in mapin.  If _end is less than 0120000 (as it was on the
distribution binary), no spl need be done at all, and the mapout can be
skipped as well if hasmap is not tested in mapin, trap and switch.
An updated system with an option to do this will probably be available.
Also, if _end is > 0120000, spl5 is usually sufficient in mapin;
unfortunately, it depends on the load order.  As long as no br6 devices
(clock included) cause access of data in this area, spl5 is all right.
The distribution used spl7 simply because it was universal and
conservative.