This lets other bits of userspace expose these various definitions too.
The function prototypes surely won't be useful in other contexts, but
the various types are, and it's not worth hiding the prototypes unless
they end up causing issues, but so long as they aren't called they
shouldn't be a problem.
Note the MAKEFS define continues to exist, but only for use in
newfs_msdos, as those sources are reused by makefs with some changed
behaviour.