Renice idle process to -20 causes hang.

While running a samba benchmark, I tried 'renice -n -20 10' (pid 10 is idle
thread) to see what would happen. Turns out that this operation isn't
illegal, and the system hung accordingly. The benchmark timed out soon
after, and the console started displaying this message:

swap_pager: indefinite wait buffer: device: ad0s1b, blkno: 776, size: 4096

over and over, although not rapidly. I'm not sure what to blame; perhaps
setpriority() should return [EINVAL] or something. I can provide more info,
upon request.

-Craig