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/config/stub_accounts.c

@@ -10,10 +10,6 @@ void
 accounts_load(void)
 {
 }
-void
-accounts_close(void)
-{
-}
 
 char*
 accounts_find_all(const char* const prefix, gboolean previous, void* context)