Add a FMT_FALSE companying FMT_NOT_EXACT

There are two types of formats that can emit several candidates for one same hash: One example is CRC32, where each crack is correct so they're not false positives - any of them is correct. An example of the other type is DMG (at least in older code) which could emit actual false candidates - they will not work for opening that DMG file!

Current code has only the false positive formats setting FMT_NOT_EXACT (after a while of scope-creep from its original idea, because the two situations really should be separated). If you want more candidates from CRC32, just add --keep-guessing option.

We could want to add a FMT_FALSE flag that allows us to separate them, and describe this better. We'd replace all current FMT_NOT_EXACT with FMT_FALSE, and then use FMT_NOT_EXACT for CRC32 and the likes.

FMT_NOT_EXACT currently prints warnings like "Note: This format may emit false positives, so it will keep trying even after finding a possible candidate." or (using --no-keep-guessing) "Note: This format may emit false positives, but we will stop after finding first".

Having both flags, those existing messages should be for FMT_FALSE instead, while the new FMT_NOT_EXACT would be more like "Note: This format has more than one correct password for each hash, (...)"

Search the format flag definitions and every use of FMT_NOT_EXACT, then trace the warning strings and the --keep-guessing behavior. Separate false-candidate handling from formats with multiple valid candidates, update the messages and flag uses accordingly, and verify both option behaviors.