The /omemo fingerprint command provided a list of all keys ever
seen for a contact but did not indicate which keys were currently "active"
(present in the latest server device list). This made it difficult for users
to identify which fingerprints actually required verification to fix
encryption issues.
Users can now easily distinguish between devices currently in use and
historical keys that no longer matter.
When encryption fails users can immediately see which "active" key is
"untrusted," allowing them to verify the correct fingerprint fast.
Function `omemo_is_device_active` will check if a fingerprint belongs
to a device currently announced by the contacts server.
Instead of adding stuff to `_shutdown()`, we can now register a shutdown
routine from the respective `init()` function of our modules.
This also has the advantage, that we're sure they're called in reverse
order from how the initialization happened.
I didn't simply use `atexit()` because POSIX says that the number of
possible callbacks is limited (min 32) and I was not sure whether
we will maybe extend this number at one point.
Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>
