Introduce our own shutdown callback mechanism.

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>

tests/unittests/omemo/stub_omemo.c

@@ -7,10 +7,6 @@ void
 omemo_init(void)
 {
 }
-void
-omemo_close(void)
-{
-}
 
 char*
 omemo_fingerprint_autocomplete(const char* const search_str, gboolean previous)