two bugs were left behind by the last refactoring, one about
consistency of the mountpoint naming (now using $TOMBNAME everywhere,
without the added .tomb extension as per #180), the other about a
missing _sudo to prefix rmdir commands.
Test suite has been updated accordingly.
This fix introduces a mandatory condition for the next update: all
tombs must be closed when upgrading the tomb script, else it will not
be able to correctly close them.
this change uses an hidden global variable within tomb to store the
decrypted key material, avoiding using one tempfile in RAM, avoiding
running the decryption more than once (which means sanity for KDF
usage) and overall simplifying the code also avoiding duplicates.
