some changes to xpt to help make unloading sim modules safer

[Matthew Jacob]

See http://people.freebsd.org/~mjacob/xpt_patches.txt

The first one come from a coworker (Chuck Tuffli) who found a hole 
xpt_create_path_unlocked.

I added a few after that to guard against overrunning some counters so 
that they would wrap.

It helped me find out what was going on much easier by adding the 
function described here: 
http://people.freebsd.org/~mjacob/xpt_path_counts.txt

Finally, http://people.freebsd.org/~mjacob/isp_patches.txt makes isp 
safe for unload as a module.

There's some ambiguity over whether it's safe to unload when a periph is 
open. Using xpt_path_counts to check with the bus reference count is > 3 
for each scsibus attached to each isp sim is a way to avoid that.

Comments?