odhcpd: add support for "ignore"

odhcpd: add support for "ignore"

This is inspired by @Kasoo's work in PR #234, but slightly different.

The "ip" option now takes "ignore" as a value , but it'll only disable
DHCPv4 for a matching client.

Similarly, the "hostid" option now also understands "ignore", which will
disable DHCPv6 for a matching client.

Of course, this is all based on client-reported MAC
addresses/DUIDs/client IDs/etc, so it's not actually a security feature.

Closes: https://github.com/openwrt/odhcpd/pull/234
Closes: https://github.com/openwrt/odhcpd/issues/198
Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/303
Signed-off-by: Álvaro Fernández Rojas <[email protected]>