Updated rusage patch

[Jeff Roberson]

I have updated the patch at:

http://people.freebsd.org/~jeff/rusage3.diff

I have incorporated much of the feedback from the earlier diff.  I have 
also changed the cpulimit code to use a callback.  This removes a PS_ flag 
and a bunch of code from mi_switch() and ast().  mi_switch() now does not 
need to touch the proc at all unless debugging is enabled.  I have tested 
the cpu limiting code as well and it seems to function properly.

My only reservation is the extra overhead of another callout per proc.

Thanks,
Jeff