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/xmpp/muc.h

@@ -82,7 +82,6 @@ typedef struct _muc_occupant_t
 } Occupant;
 
 void muc_init(void);
-void muc_close(void);
 
 void muc_join(const char* const room, const char* const nick, const char* const password, gboolean autojoin);
 void muc_leave(const char* const room);