odhcpd: rename [lease_cfg|dhcpv4_lease]->ipaddr to ipv4

odhcpd: rename [lease_cfg|dhcpv4_lease]->ipaddr to ipv4

"ipaddr" doesn't say if it's IPv4 or IPv6, and we want to be clear in a
dual-stack codebase.

And at the same time, use a struct in_addr to store the IPv4 address,
which is just less confusing.

Finally, remove all uses of "inet_ntoa()", which is deprecated.

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/306
Signed-off-by: Álvaro Fernández Rojas <[email protected]>