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.
This commit re-organizes all the source distribution contents to
present users with the simple script, while moving the rest in extras.
Also autoconf/automake scripts were removed, back to minimalism.
The rationale of this change is that Tomb really only consists of a
script and users with no extra needs should just be presented with
it with no need for anything else. Any other thing on top of the Tomb
script is an extra and can be even distributed separately or integrated
in distributions.
