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/common.h

@@ -196,5 +196,6 @@ gchar* get_expanded_path(const char* path);
 char* basename_from_url(const char* url);
 
 gchar* prof_get_version(void);
+void prof_add_shutdown_routine(void (*routine)(void));
 
 #endif