Add a --keep-guessing option, that sets FMT_NOT_EXACT for any
format and thus lets us search for plaintext collisions.

Since we are out of format flags, I dropped FLG_MKPC and used
that one for this instead (because --mkpc requires an option).
OldOffice format: If we cracked a hash and are in FMT_NOT_EXACT
mode (ie. --keep-guessing option was used), store a meet-in-the-
middle value for faster cracking.

This does not yet work after a resume because the MITM value is
not stored.