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/command/cmd_defs.h

@@ -42,7 +42,6 @@
 #include "ui/ui.h"
 
 void cmd_init(void);
-void cmd_uninit(void);
 
 Command* cmd_get(const char* const command);
 GList* cmd_get_ordered(const char* const tag);