Compute the checksum of the file.

[mario-campos]

I've noticed that at least one XDCC sender will send the MD5 hash digest in a separate message. It would be nice to compute the hash of the file once the download completes and compare it to what was sent to us.

Obviously, since not every XDCC sender may send us this hash, we should provide a way to opt-out (or maybe opt-in?).

Bonus points: the checksum calculator can run in a separate thread while the file is transferring.