Perform MTU discovery on UDP sockets and respect path MTUs.

[jpcsmith]

Although SCION reports path MTUs, this can be seen as an upper bound. NICs and other networking setups may impose additional MTU restrictions. The socket should therefore perform some form of MTU discovery to determine the MTU usable on the local network.

This may also need to be adapted when the underlay_next_hop changes for different paths, as the path to the new underlay may go out a different interface.