mkfs.minix: add v3 support

Hi,

The following patches add support for the minix3 filesystem in mkfs.minix. I have been able to successfully try this out on different disks, loading different types of files (images, text, video, etc.). I have also verified that stat(2) works fine on this and the regression tests for util-linux pass. Since I am not a fs expert please try this out thoroughly and let me know your opinion. Despite the ugliness of the code (for years now), I tried not to be too invasive and not rewrite large parts of it.

With this new version we can
* have up to 60 characters for filenames.
* specify the block size to be used.

Additionally, four options were added:
* -1, -2, -3 to specify what version of the fs to create
* -B to specify the block size

Thanks,
Davidlohr