Introduce `rt_lock' rwlock(9) and use it instead of kernel lock to

Introduce `rt_lock' rwlock(9) and use it instead of kernel lock to
serialize arpcache() and arpresolve(). In fact, net stack already has
sleep points, so the rwlock(9) is better here because we avoid
intersection with the rest of kernel locked paths. Also this new lock
assumed to use to route layer protection instead of netlock.

Hrvoje Popovski had tested this diff and found no visible performance
impact.

ok bluhm@