Implement check for demo checksum

Poking around in the replay files, I noticed that the replay checksum is never checked when a replay is loaded. While the speedrun community has made their own tool to verify these, it seems worthwile considering verifying this in vanilla. As the checksum may be incorrect for non md5 builds, I have also enabled skipping the check with the -force argument.

Issues/Open questions:

• I have noticed that on builds without the md5 library the checksum is set to 0 bytes. How should that case be handled? Is that even the case in any of the official builds? How does automatic addon loading work in that case?
• While I made a reasonable effort to check this, is the length detected correctly in every case?