Kernel preemption

[Andrew Doran <ad%netbsd.org@localhost>]

Hi,

The below patch implments in-kernel preemption for the i386 port. By default
LWPs will only preempt in kernel if they are in a real-time priority band.
The priority level is tunable via sysctl. Setting it to zero seems to shake
out a lot of interesting bugs.

It needs a bit more work, for stuff like FPU state, and I also want to
extend it to amd64. I think it should not be enabled by default until the
bugs it exposes are shaken out. Any comments?

http://www.netbsd.org/~ad/preempt.diff

Thanks,
Andrew