Introduce a pointer to const variable gw, which points either at the
same place as dst, or to the sockaddr in the routing table.

The const constraint of gw makes us safe from modifing routing table
accidentially. And "onstantness" of dst allows us to remove several
bandaids, when we switched it back at &ro->ro_dst, now it always
points there.

Reviewed by:	rrs