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/plugins/plugins.h

@@ -113,7 +113,6 @@ GSList* plugins_unloaded_list(void);
 GList* plugins_loaded_list(void);
 char* plugins_autocomplete(const char* const input, gboolean previous);
 void plugins_reset_autocomplete(void);
-void plugins_shutdown(void);
 
 void plugins_free_install_result(PluginsInstallResult* result);
 
@@ -129,7 +128,6 @@ gboolean plugins_reload(const char* const name, GString* error_message);
 void plugins_reload_all(void);
 
 void plugins_on_start(void);
-void plugins_on_shutdown(void);
 
 void plugins_on_connect(const char* const account_name, const char* const fulljid);
 void plugins_on_disconnect(const char* const account_name, const char* const fulljid);