Implement encrypted cdrom ejection

This allows the cdrom to be copied onto the outer filesystem (mounted in
/outerfs) without storing unencrypted gpg keys there.

e.g.:

  samizdat-eject.sh /outerfs/samizdat.iso

This was necessary because the other method probably causes btrfs deadlocks.

We do end up copying data twice this way (or three times, probably --
if the ISO is saved), but not _from the cdrom_.  And we get to eject
immediately after the first copy.  Future copies will be from the hard
drive.  Not too bad.

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