Allow checksum verification to be skipped

On Windows, checksum verification sometimes take a while (mostly noticed on low-end computers), causing players to disconnect from Beyond All Reason while loading a map (as such when they eventually load it looks like they're alone in a battle even thoug they're not). Even though checksum verification is a *very* good thing to have, the time it takes to load a map due to it makes making this check optional a necessity.

No file, test, or entry point is named in the issue. Start by locating the Windows map-loading path that performs checksum verification and identify how an optional user setting could reach it. Done means checksum verification can be skipped when chosen, while the existing verification behavior remains available and loading no longer waits for that check in the skip case.

Written by the indexing model from the issue text.