Use .To4() on serverId

[tfheen]

The server id should be four octets, per RFC1533, so call .To4 on it
to ensure we have a v4 address and not a mapped v4 address.

[tfheen]

I wasn't entirely sure if you wanted to keep the API, .To4() can return nil if serverId is a v6 address, in which case the ReplyPacket should probably fail. Not using .To4() means you can end up sending a 16 octet long ServerIdentifier, which doesn't work with all DHCP clients (at least the ISC DHCP client is unhappy with it).

[krolaw]

I'm going to think about this some more.
If IPv4 nochange.
If mapped v4 corrects a potential bug.
If IPv6 goes from invalid serverId (too long) to invalid serverId (zero length).

However behaviour would be inconsistent between ReplyPacket and .AddOption which could make a bug harder to find.