Fix filetransfers, invalid port nums, and other bullshit that was breaking this server

- Fix resending of packets, sometimes they would retry 3 times in half a second (which is wrong), default wait time is 5 seconds
- Fix bound sockets being removed from the multiplexer, causing that "we read one packet then can't receive anything anymore" bug again
- Defaulted to using the IPv6 bind address since the linux TFTP client likes doing TFTP6, may or may not be reverted
- Fixed a bug in GetPort which invalidly changed the endianness of the port integer
- Created the GetAddress function which like GetPort, gets the IPv4/IPv6 address as a temporary static string