odhcpd: rename iface->addr4 -> iface->oaddrs4

odhcpd: rename iface->addr4 -> iface->oaddrs4

Rename iface->addr4 to iface->oaddrs4, to make it clearer from a cursory
glance that this isn't e.g. "struct in_addr" addresses but a number of
"struct odhcpd_ipaddr" addresses.

At the same time, rename iface->addr4_len to iface->oaddrs4_cnt, to make
it clear that this is a count and not e.g. an allocation length.

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/320
Signed-off-by: Álvaro Fernández Rojas <[email protected]>