[cmds] Finish fsck-dos port to ELKS, add to distribution

[ghaerr]

As discussed in #1819 (comment), the excessive memory usage in fsck-dos was identified, where each directory entry found was allocating 256 bytes for a filename. Since ELKS only handles 26 characters max for FAT, this was shortened, and fsck-dos is no longer running out of memory on 1440k floppy disks.

This PR adds fsck-dos to the 1440k distribution, for testing:

$ fsck-dos -pf /dev/fd1    # run in "preen" mode to quickly force fix small errors
$ fsck-dos -yf /dev/fd1    # force check and answer yes to all errors
$ fsck-dos -nf /dev/fd1    # force check and answer no to all errors

I'll look for a man page for ELKS, but this one from OpenBSD seems to describe its use fairly accurately.

CURRENTLY USE AT YOUR OWN RISK - NOT TESTED THOROUGHLY!