#755 Validate Gateway address

[Wayne Osborn]

It is possible to enter an invalid gateway address to a LAN interface in static mode. I suggest a validity checking when saving LAN settings, ensuring the entered gateway address is on the same subnet as the interface address.

[Daisuke Aoyama]

If IP is only one, it should be same subnet.
However, for multiple NICs, IPs, it should be made careful.