Support connecting to networks with misconfigured subnet masks.

In K and earlier, we would connect to a network where the gateway
was not covered by the subnet mask of the IP address. This is an
invalid configuration, but it used to work, and other OSes appear
to accept it too, so support it.

Bug: 19067207
Change-Id: I822e1d754b336691b675438eefa959a3d75fd07b