Implement Path MTU Discovery as in libtorrent

this µTP library appears to hard code MTU to 1400 bytes. This will often work okay with home internet over UDP4 and UDP6, but may cause problems when people use tunnels like VPNs, especially if they chain several tunnels or proxies together for extra privacy. Supporting MTU discovery will also be important if users plug UTP in to other networks such as Bluetooth. An MTU of 1000 can be more appropriate for Bluetooth PAN as in phone internet tethering.

With recent news on dragnet surveillance and the renewed importance of leaking and anonymous communication, I feel it is important our network libraries have good support for very private users with multiple layers of proxies.

http://www.libtorrent.org/utp.html#path-mtu-discovery

I open this issue mainly as a discussion. I am not aware of any way in Node to disable the Don't Fragment attribute on outgoing UDP packets, so we cannot implement the libtorrent solution, at least not with native dgram.

Start with the linked libtorrent Path MTU Discovery reference and Node's native dgram UDP API; the issue specifically notes that disabling the Don't Fragment attribute may not be available there. Determine the discovery approach for chained tunnels, proxies, and Bluetooth PAN, with completion meaning the library no longer relies on a fixed 1400-byte MTU.