mpi@ introduced a ph_cookie in mbuf packet headers that can be used

mpi@ introduced a ph_cookie in mbuf packet headers that can be used
by a subsystem to stash some state while the mbuf gets queued. eg,
net80211 uses it to keep track of the wireless node associated with
a packet before submitting it to a drivers snd queue for transmission.

this makes pipex use ph_cookie to keep track of the pipex session
associated with a packet before submitting it to the softint queues.
this lets us get rid of an mbuf tag type, and avoids the cost of
tag allocation.

ok yasuoka@