Auto-select default data filesystem - #97

The user can always choose the filesystem explicitly using:

Also, most ISO images are already larger than 4 GB, and FAT32 does not support files larger than 4 GB. I'll leave this topic open for further discussion.

exFAT would remain the default as long as mkfs.exfat is installed. If it isn't installed, MultiOS-USB currently bails out when invoked with the default settings. This effectively makes mkfs.exfat a hard dependency, even though exFAT isn't actually required for MultiOS-USB to work; it's merely recommended.

That's exactly what this PR proposes: if mkfs.exfat is available, use it; if it isn't, fall back to the next-best option (FAT32).

This is also why most major package management systems differentiate between Requires (hard dependency), Recommends (recommended optional dependency; installed by default), and Suggests (suggested optional dependency; not installed by default).

This effectively makes mkfs.exfat a hard dependency

Please don't.