rtsock: Fix !SO_USELOOPBACK support in route_output()

rtsock: Fix !SO_USELOOPBACK support in route_output()

Old BSD code assumes that rts_input() is synchronous, so clear/set
sp_family could be used to skip the current rawcb.  However, our
rts_input() is asynchronous, thus the clear/set sp_family trick no
longer takes any effect.

Reimplement the rawcb skipping in the following way:
- Record the address of rawcb to be skipped in netmsg.
- Extend raw_input() interface to accept a 'skip' argument.  At the
  beginning of the rawcb list iteration, check the address of the
  current rawcb against the 'skip'.