Clean up the netlock macros. Merge NET_RLOCK_IN_SOFTNET and

Clean up the netlock macros.  Merge NET_RLOCK_IN_SOFTNET and
NET_RLOCK_IN_IOCTL, which have the same implementation.  The R and
W are hard to see, call the new macro NET_LOCK_SHARED.  Rename the
opposite assertion from NET_ASSERT_WLOCKED to NET_ASSERT_LOCKED_EXCLUSIVE.
Update some outdated comments about net locking.
OK mpi@ mvs@