initial rusage patch.

[Jeff Roberson]

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

I'm providing this patch for discussion only.  I've just implemented 
enough that you can see the fallout from this change.  I have not yet 
tested enough to say that this is perfect.  I have also not yet fixed the 
RLIMIT_CPU check in mi_switch().

You can see, however, that with this change there is no access to struct 
proc in mi_switch() except for INVARIANTS and KTR.  Aside, of course, for 
the rlimit that needs to move anyway.

You can also see, that most access to rusage are done through fewer 
indirects and to local memory.  The storage impact of struct proc doesn't 
change as pstat can be reclaimed as it could before.

Furthermore, the only time we need locks is in rufetch() where we 
aggregate the threads counters and rusage structs into one allocated by 
the caller.  Doing this aggregation less frequently means we're touching 
struct proc less frequently.

In this patch the scheduler lock protects this aggregation.  In my 
threadlock diff this will be protected by the per process spinlock and the 
thread lock.  However, in most places that we aggregate with calcru() 
we're grabbing a spinlock anyway.  So it is not so expensive to grab 
another.

Thanks,
Jeff