Implement rollover

The current implementation only allows transfers of a maximum of 87 MB.

Rollover is a nonstandard option, but most clients implement it. It basically resets the block number counter back to 0 after it reaches 2^16. This breaks multicast TFTP though, because after a rollover, the second TFTP server will not know at which position in the file it is supposed to seek. 

This will allow transfering files of any size.