dhcp4: prepend gateway device routes if needed (bnc#887586)

When a route gateway is not on the same network as the IP
address, e.g. because the server provided us a IP/32, and
there are no matching device routes provided, prepend one.
address: omit brd when equal to address (bnc#887586)
route: check gw address to set scope (bnc#887586)

Fixed guess scope helper to also check the if route gateway
address is specified, not only if it's family is unspec to
not set device routes with universe scope.