[misc] fix FAT32 cluster boundaries and add cheat mode to force DD

* Closes #310
* Alt-I can now be used to disable ISO support. This is meant to be
  used for dual ISO/DD images so that DD (disk image) mode will be used
* Also issue a message in the log to indicate whether slow or quick format is used
* Also change the cheat mode keys for force update and proper unit to C and U