dhcpv4: create struct dhcpv4_lease

dhcpv4: create struct dhcpv4_lease

This makes it easier to understand what is actually going on, makes the
structs clearer and avoids layers of indirection like using free
callbacks. I understand the original motivation for unifying the struct
for DHCPv4 and DHCPv6, but it's more hassle than it is worth.

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/292
Signed-off-by: Álvaro Fernández Rojas <[email protected]>