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.
Merge pull request #325 from mtomaschewski/dhcp4-route