livecd-iso-to-disk: Fix labeling of target device filesystem.
    
    While trying to preserve the device filesystem label across a
    format change, a long name may be inappropriate for the new format.
    
    mkfs.fat -n truncates a name longer than 11 bytes, and
    mkfs.ext[34] -L truncate a label to 16 bytes.
    
    Use mkfs.fat instead of the mkdosfs symbolic link to it.
    Also clarify some label handling and
      remove some useless exit status checks.
    
    (This commit fixes rhbz:1444530 livecd-tools/issue/59.)