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/xmpp/stub_xmpp.c

@@ -47,10 +47,6 @@ session_disconnect(void)
 {
 }
 void
-session_shutdown(void)
-{
-}
-void
 session_process_events(void)
 {
 }
@@ -477,11 +473,6 @@ caps_lookup(const char* const jid)
     return NULL;
 }
 
-void
-caps_close(void)
-{
-}
-
 void
 caps_destroy(EntityCapabilities* caps)
 {