Add better routes for secondary DNS

We'd been doing no-gateway hostroutes for dns servers on secondary nets, but on
some devices (multi-homed stingray) this is a problem.  Add gateway-ed hostroutes
instead so the BP can do it's nonstandard "magical" demultiplexing.

bug:5011392
Change-Id: Ia48f69c8ddf2a37cfb8f014f078f96bf601d2ddb