Major update to the way synchronization is done in the kernel. Highlights
include:

• Mutual exclusion is used instead of spl*(). See mutex(9). (Note: The alpha port is still in transition and currently uses both.)

• Per-CPU idle processes.

• Interrupts are run in their own separate kernel threads and can be preempted (i386 only).

Partially contributed by: BSDi (BSD/OS)
Submissions by (at least): cp, dfr, dillon, grog, jake, jhb, sheldonh