Decrease netlock pressure in pppx(4).

Decrease netlock pressure in pppx(4).

Push netlock down to pppx_add_session(). The 'pppx_if' structure has
the `pxi_ready' member to prevent access to incomplete `pxi', so we
don't need to hold netlock during all initialisation process. This
removes potential PR_WAITOK/M_WAITOK allocations impact on packet
processing. Also this removes relock dances around if_attach() and
if_detach() calls.

Do not grab netlock for FIONREAD. mbuf(9) queue doesn't rely on it.

Do not grab netlock around pipex_ioctl() call. pipex(4) has its own
protection and doesn't rely on netlock. We need to unlink  pipex(4)
session before destroy associated `pxi', it can't be killed
concurrently. Also this stops to block packet processing when npppd(8)
periodically does PIPEXGCLOSED ioctl(2) commands.

The dummy FIONBIO case doesn't require any lock to be held.

The netlock remains to be taken around pppx_del_session() and
pppx_set_session_descr() because pppx(4) data structures rely on it.

Tested by Hrvoje Popovski.

ok yasuoka@