Fix logic error in handling resolveGateway()

* Use gateway address to trigger resolution rather than network address
* Only reschedule handling a route if resolveGateway() returns 0
* Force initiateND() to fail on IPv6 (noisily) since that just fails for now

Change-Id: I604c9359125f2470294aeaaac59fe0ae4121071a

Signed-off-by: Mark Lentczner <mzero@google.com>
Signed-off-by: Joe Stringer <joe@wand.net.nz>