n-dhcp4: discard NAKs from different servers in SELECTING

I got a report of a scenario where multiple servers reply to a REQUEST in SELECTING, and all servers send NAKs except the one which sent the offer, which replies with a ACK. In that scenario, n-dhcp4 is not able to obtain a lease because it restarts from INIT as soon as the first NAK is received. For comparison, dhclient can get a lease because it ignores all NAKs in SELECTING.

Arguably, the network is misconfigured there, but it would be great if n-dhcp4 could still work in such scenario.

According to RFC 2131, ACK and NAK messages from server must contain a server-id option. The RFC doesn't explicitly say that the client should check the option, but I think it's a reasonable thing to do, at least for NAKs.

This patch stores the server-id of the REQUEST in SELECTING, and compares it with the server-id from NAKs, to discard other servers' replies.