Directory claiming and ephemeral filesystems.

Two new related tools.

  * `mount-ephemeral' creates (and removes) a temporary filesystem,
    encrypted using a fresh random key so the contents are irretrievably
    lost when the host reboots or the power fails.

  * `claim-dir' is a `userv' service which allows users to claim
    directories in a shared filesystem without the hazardous
    free-for-all that results from world writability with a sticky bit.

These go in their own separate Debian package.  There's no direct link
between the two, but bundling them together provides a hint regarding
possible applications.