Suggestion for possible mitigation of the tape corruption problem in #446

**Is your feature request related to a problem? Please describe.**
I've just experienced for the first time the tape corruption mentioned in #446. This was my own fault, and the first time I've run into it, because previously when I've tried to eject the tape too early, I've always used ``mt -f /dev/nst0 rewoffl`` instead of ``mt -f /dev/st0 rewoffl`` to eject the tape, and as far as I can tell, this is not a problem when using the nst device, which does not rewind the tape when closing the device, and it's the rewind behavior that specifically causes this problem. When using nst, I just get an I/O error, but the tape is not corrupted, and all I need to do is try ejecting it again. However, I think it's still confusing to the average user that calling the fairly standard system "eject tape" command can irreparably corrupt a tape with no warning.

I don't like using the ``eject`` mount option to ``ltfs``, because I often run tape backups remotely, at night or on weekends, and it happens frequently that I forget to copy something to the tape before umounting it. If it automatically ejects on unmount, I need a person to be physically present to re-insert it in the drive.

**Describe the solution you'd like**
I think this problem would be largely avoided by simply having ``umount`` not return until ltfs has finished writing everything to the tape and the tape is safe to rewind, eject, or do whatever to. ``umount`` returning almost immediately, but the actual flushing process to tape running in the background for an indeterminate amount of time that can only be discovered by peeping at process lists is not user friendly or intuitive. It's also incongruent with how ``umount`` normally works on Linux systems; when running ``umount`` on a filesystem on an external disk drive, for instance, ``umount`` does not return until all data is flushed to disk and the filesystem properly updated (which in some cases on slow devices can take a while), and when ``umount`` returns, it's a sign that the external disk drive is safe to unplug. This also mimics behavior in GUIs and other operating systems where the drive's icon doesn't disappear from the desktop when unmounting until all data has been successfully flushed.

I think making ``umount`` wait until the tape has finished flushing would be the preferred behavior, reducing the chance of mishaps, be more intuitive, and would also make scripting, etc., easier. Additionally, I think the documentation should strongly recommend only using the ``/dev/nstX`` devices for any manipulation of the tape drive, instead of the ``/dev/stX`` devices, since the ``/dev/nstX`` devices seem to not risk corruption of the tape.

Start by investigating ltfs's unmount handling and the flush process described in the issue, then compare the /dev/stX and /dev/nstX behaviors. Done means umount does not return before tape flushing is complete and the documentation clearly recommends the safer device usage.

Written by the indexing model from the issue text.