Look for existing checksum files on disk

[robbat2]

Look for classic checksum files on disk, and import/validate those.

Some common patterns: *sfv, *DIGEST*, *DIGESTS*, *CHECKSUMS*

The most common formats on these will be:

• SFV: $FILENAME $SFV
• BSD checksums (openssl dgst default) MD5(filename) = ...
• coreutils checksums ... *filename (the * is optional). Watch out for digests of the same length but unknown type.