Implement per-socket `so_lock' rwlock(9) and use it to protect routing

Implement per-socket `so_lock' rwlock(9) and use it to protect routing
(PF_ROUTE) sockets. This can be done because we have no cases where one
thread should lock two sockets simultaneously.

Against the previous version rtm_senddesync_timer() execution was moved
to process context.

Also this time `so_lock' used for routing sockets only but in the future
it will be used to other socket types too.

tested by claudio@

ok claudio@ bluhm@