Introduce lightweight PMTU probe replies

When replying to a PMTU probe, tinc sends a packet with the same length as the PMTU probe itself, which is usually large (~1450 bytes). This is not necessary: the other node wants to know the size of the PMTU probes that have been received, but encoding this information as the actual reply length is probably the most inefficient way to do it. It doubles the bandwidth usage of the PMTU discovery process, and makes it less reliable since large packets are more likely to be dropped.

This patch introduces a new PMTU probe reply type, encoded as type "2" in the first byte of the packet, that indicates that the length of the PMTU probe that is being replied to is encoded in the next two bytes of the packet. Thus reply packets are only 3 bytes long.

(This also protects against very broken networks that drop very small packets - yes, I've seen it happen on a subnet of a national ISP - in such a case the PMTU probe replies will be dropped, and tinc won't enable UDP communication, which is a good thing.)

Because legacy nodes won't understand type 2 probe replies, they are only used with nodes that are using the experimental protocol.