Teach devd about usb "release" field. [PATCH]

This one was partially committed by Ian earlier but the entries that 
used the release field were not since devd didn't know about it.  The 
patch at the end of this PR teaches devd about the release field so 
we should be able to kill off usbd.conf/usbd.
http://www.freebsd.org/cgi/query-pr.cgi?pr=73799

Thanks,
-- 
Anish Mistry