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/pgp/gpg.h

@@ -55,7 +55,6 @@ typedef struct pgp_pubkeyid_t
 } ProfPGPPubKeyId;
 
 void p_gpg_init(void);
-void p_gpg_close(void);
 void p_gpg_on_connect(const char* const barejid);
 void p_gpg_on_disconnect(void);
 GHashTable* p_gpg_list_keys(void);