Unifying /dev/{mem,kmem,zero,null} implementations

[Joerg Sonnenberger <joerg%britannica.bec.de@localhost>]

Hi all,
http://www.netbsd.org/~joerg/common-mem.diff
unifies the implementation of /dev/{mem,kmem,zero,null} in MI code and
makes it MPsafe. This is useful as writing to /dev/null is not that
unknown :-)

This needs quite a bit attention from portmasters and testing:
- the address returned by mmap of /dev/mem is not consistent on all
ports, I have no idea if it works on those where atop() doesn't exist.
- on i386, the special /dev/io semantic for COMPAT_10 and COMPAT_FREEBSD
is lost at the moment -- do we want to keep that? It would mean another
platform hook.
- the access permissions for raw memory are inconsistent and should be
changed to use kauth on all platforms.

Over all, the patch adds 1419 lines and removes 6271.

Comments?

Joerg