The "ifq_set_maxlen(..., 1);" hack we use to enforce pipex(4) related

The "ifq_set_maxlen(..., 1);" hack we use to enforce pipex(4) related
(*if_qstart)() be always called with netlock held doesn't work anymore
with PPPOE sessions.

Introduce `pipex_list_mtx' mutex(9) and use it to protect global pipex(4)
lists and radix trees.

Protect pipex(4) `session' dereference with reference counters, because we
could sleep when accessing pipex(4) from ioctl(2) path, and this is not
possible with mutex(9) held.

ok bluhm@