Part of a fix for [FDOS/label#17]

Part of a fix for [FDOS/label#17]

If the BPB is either v4.1 or v7 long, then its volume label field
should be written.

Note:
  This site https://jdebp.uk/FGA/bios-parameter-block.html suggests that
it is perfectly valid to have a v7 long BPB with a FAT12 or FAT16
filesystem, although more usually it's used for FAT32. However I can't
see any confirmation of this elsewhere, haven't seen an example of this
in the wild, and have no means of generating a test article. More
importantly since we are writing to the filesystem, it's important to
not have any false positives or we could cause corruption. So for now
this combination, should it exist, will not be updated. See the
discussion here dosemu2/fdpp#202.

Part of a fix for [FDOS/label#17]