Use btrace(8) to debug reference counting. dt(4) provides a static

Use btrace(8) to debug reference counting.  dt(4) provides a static
tracepoint for each type of refcnt we have.  As a start, add inpcb
and tdb refcnt.  When the counter changes, btrace may print the
actual object, the current counter, the change value and optionally
the stack trace.
discussed with visa@; OK mpi@