1. IPv6 support

   See pull request #365 and https://github.com/PilzAdam/minetest/tree/ipv6-revised

   Needs testing on windows?

2. "listen" config setting

   This specifies a list of IP addresses or network interfaces to bind and listen on
   when starting a server.

   Examples:
   "0.0.0.0" means to listen on all IPv4 addresses (default).
   "::" means to listen on all IPv6 addresses.
   "0.0.0.0, ::" means a combination of the above.
   "eth0, wlan0, lo" means to listen on these 3 interfaces (but not eth1, for example)

   (This eliminates the need for the ipv6_server option.)

   Setting an interface name here works by reading the list of IP addresses of the
   interface when starting the server, and listening on all of them. This requires
   some ioctl magic on Linux and WSAIoctl on Windows (which are the only operating
   systems I can test, code to do this on BSD, Mac OS X and other systems would be
   welcome).
   Winsock: http://tangentsoft.net/wskfaq/examples/getifaces.html

3. Set proper source address.

   The server should use the address on which the initial packet of a connection
   was received as the source address in the packets it sends. Right now, if the
   destination is reachable via multiple interfaces or an interface with several
   IPs, the choice of the source address is left to the OS. In case this address
   is not the address the client connected to, it won't know what to do with the
   packets it receives and will time out while trying to connect.

4. In-process datagram pipe

   In singleplayer mode, localhost UDP sockets should not be used since they can
   be slow. Instead store datagrams in a buffer when sending. Read from the same
   buffer when reading on the other end.