#825 dnsmasq not respecting new fixed leases

[Does it matter?]

If I edit a dynamic lease in the UI, assign the device a different IP address and save it, dnsmasq ACKs the next request from that device for a renewal, so the new address is never delivered.

Many devices "remember" the last IP address, so even a reboot won't force it to pick up the new address. Frequently I've had to reset devices so they pick up the new IP address, which means I have to then reconfigure them. A substantial waste of time and effort.

dnsmasq should NAK a request from a device for an address renewal if there is a fixed lease for that MAC and the requested address isn't the same as the fixed address.