Switch alpha to MI mplock code.

remove __mp_release_all_but_one(), unused since sched_bsd.c rev 1.92
ok claudio@

Change __mp_lock_held() to work with an arbitrary CPU info structure and
extend ddb(4) "ps /o" output to print which CPU is currently holding the
KERNEL_LOCK().

Tested by dhill@, ok visa@

Work in progress work towards SMP, heavily based upon NetBSD. The MP kernel
will boot multiuser, but will deadlock under load, and I can't find my
mistake yet.