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>

src/omemo/omemo.h

@@ -61,7 +61,6 @@ typedef struct omemo_key
 } omemo_key_t;
 
 void omemo_init(void);
-void omemo_close(void);
 void omemo_on_connect(ProfAccount* account);
 void omemo_on_disconnect(void);
 void omemo_generate_crypto_materials(ProfAccount* account);