Workaround older blkid not distinguishing between FAT16 and FAT32 (#743181)

Older versions of blkid don't correctly distinguish between FAT16 and
FAT32 file systems when overwriting one with the other.  This effects
GParted too with these file systems on whole disk devices where only
blkid is used to recognise the contents.  See previous fix for why only
blkid is used in this case:
    Avoid whole disk FAT being detected as MSDOS partition table
    (#743181)

Example:

    # blkid -v
    blkid from util-linux 2.20.1 (liblkid 2.20.0, 19-Oct-2011)
    # mkdosfs -F16 -I /dev/md1
    # blkid | fgrep md1
    /dev/md1: SEC_TYPE="msdos" UUID="7C23-95D9" TYPE="vfat"
    # mkdosfs -F32 -I /dev/md1
    # blkid | fgrep md1
    /dev/md1: SEC_TYPE="msdos" UUID="7F93-98F4" TYPE="vfat"

So blkid recognised the UUID changed but didn't remove the SEC_TYPE for
the FAT32 file system.  See FS_Info::get_fs_type() as it uses this to
distinguish between FAT16 and FAT32.  This is a caching update bug in
blkid, because telling blkid not to use the cache gets the right
results:

    # blkid -c /dev/null | fgrep md1
    /dev/md1: UUID="7F93-98F4" TYPE="vfat"

With testing determined that blkid from util-linux 2.23 and later are
not affected and earlier versions are affected.  Mostly recently known
affected distribution is Ubuntu 14.04 LTS with util-linux 2.20.1.

The straight forward fix would be to instruct blkid to not use its cache
with 'blkid -c /dev/null'.  But using blkid's cache is needed to prevent
blkid hanging for minutes when trying to access a non-existent floppy
drive when the BIOS is set incorrectly.  See commit:

    18f86315
    Fix long scan problem when BIOS floppy setting incorrect

Instead, when using an older affected version of blkid and when blkid
cache reports a vfat file system, run blkid again bypassing the cache.
The device is known to exist and contain a vfat file system, just not
whether it is a FAT16 or FAT32 file system, so can't be a non-existent
floppy device and won't hang.

Bug 743181 - Add unpartitioned drive read-write support