luci-mod-network: issue with breakdown of dnsmasq after duplication of static IP was fixed

Before this commit, assigning the same static IP address to two
different hosts disabled dnsmasq.

Logic of adding a new static lease was modified. If user try to assign a
new MAC address to already reserved IP, old lease will be modified (list
of MAC addresses will be extended by new MAC) instead of creation a new lease with the same IP.

Signed-off-by: Oleksandr Pastushkov <oleks.pastushkov@gmail.com>