Teach pf and ipfw to use read locks in inpcbs write than write locks

Teach pf and ipfw to use read locks in inpcbs write than write locks
when reading credential data from sockets.

Teach pf to unlock the pcbinfo more quickly once it has acquired an
inpcb lock, as the inpcb lock is sufficient to protect the reference.

Assert locks, rather than read locks or write locks, on inpcbs in
subroutines--this is necessary as the inpcb may be passed down with a
write lock from the protocol, or may be passed down with a read lock
from the firewall lookup routine, and either is sufficient.

MFC after:	3 months