Route cache function returns hit or miss.

Route cache function returns hit or miss.

The route_cache() function can easily return whether it was a cache
hit or miss.  Then the logic to perform a route lookup gets a bit
simpler.  Some more complicated if (ro->ro_rt == NULL) checks still
exist elsewhere.
Also use route cache in in_pcbselsrc() instead of filling struct
route manually.

OK claudio@