Pull up following revision(s) (requested by knakahara in ticket #332):
sys/net/if_pppoe.c: 1.127-1.128
sys/net/if_pppoe.h: 1.15
sys/net/if_spppsubr.c: 1.170-1.171
sys/net/if_spppvar.h: 1.21-1.22
Integrate two locks used to protect PPPoE softc. Contributed by s-yamaguchi@IIJ.
PPPOE_SESSION_LOCK protects variables used in PPP packet
processing, on the other hand PPPOE_PARAM_LOCK protects
the other variables used to establish a PPPoE session id.
Those locks isn't acquired in the same time because the
PPP packet processing doesn't work without PPPoE session id.
By the reason, the locks can be integrated into PPPOE_LOCK.
Add locking notes later.
--
sppp_lock is changed from mutex to rwlock now. Contributed by s-yamaguchi@IIJ.
Add locking notes later.
--
Add a locking notes for if_pppoe
--
Add a locking notes for if_spppsubr
--
fix no INET6 build.

(snj)