Some changes for the XBPS libfetch's frontend code.

1) Raise the cache connection settings to more sane defaults (8->6, 16->2).
2) Introduce xbps_fetch_unset_cache_connection() to drop the caches.

bin/xbps-bin/main.c

@@ -120,6 +120,7 @@ static void
 cleanup(int signum)
 {
 	xbps_regpkgs_dictionary_release();
+	xbps_fetch_unset_cache_connection();
 	exit(signum);
 }