#10 Use the network mask when checking isAdressValidForIf

[Mike C. Fletcher]

This patch allows for netmasks where the ip address is 0.0.0.0/bits by using the netmask in the check for isAddressValidForIf. It's a one-line patch, but it could be done in config.c (that is, the address could be masked on reading against the bitmask) if the run-time mask-check is a problem.