stop scheduler in panic context

[Andriy Gapon]

I would like to present the following diff for review and discussion:
http://people.freebsd.org/~avg/stop_scheduler_on_panic.diff

The idea is to stop scheduler in a panic context and to provide a special
environment for the only running thread, the one that called panic(9).

I tried to make this diff as minimal as possible, it doesn't include changes that
I consider to be useful improvements and [even] bug fixes, but which generated
controversy in non-public discussions.

If there is no negative feedback within next few days, then I plan to post the
patch to current@ to solicit some testing.  I will definitely wait for positive
feedback before committing this change.  I hope that I will be able to sneak it
into the 9 release (unless there are objections to this).

Thank you!
-- 
Andriy Gapon