This patch adds support for multiprocessor kernel debugging,
documented in /sys/doc/acidmach.ms.

Other sundry changes include:

Each kernel has been updated to emit the full path on panic.  This
behavior can be changed by using the *nodumppath configuration option.
This greatly reduces time spent debugging via ktrace.  Support for
*nodumpstack was extended to other kernels as well.

libmach was updated to support stack generation past interrupt frames
on i386.

A number of small issues existed in the panic code path, which could
lead to deadlock after an rdb session.  These were corrected for each
kernel and included minor cleanup of shutdown behavior in the event of
panic.

NMI handling for i386 was also reworked to support immediate
debugging.  Vestigal NMI setup was also removed in favor of the new
interface.