Make sure the certificate cache is flushed when plugins are unloaded.

This avoids segmentation faults when plugins implementing cert_t are
already unloaded when the cache is flushed during destruction.

src/pluto/plutomain.c

@@ -852,6 +852,7 @@ void exit_pluto(int status)
 	delete_lock();
 	options->destroy(options);
 	pluto_deinit();
+	lib->credmgr->flush_cache(lib->credmgr, CERT_ANY);
 	lib->plugins->unload(lib->plugins);
 	libhydra_deinit();
 	library_deinit();