Prevent unload of modules which implement an Optional API.

Once an Optional API module is loaded it should stay loaded.  Unloading
an optional API module runs the risk of a crash if something else is
using it.  This patch causes all optional API providers to tell the
module loader not to unload except at shutdown.

ASTERISK-27389

Change-Id: Ia07786fe655681aec49cc8d3d96e06483b11f5e6

res/res_smdi.c

@@ -1407,6 +1407,10 @@ static int _unload_module(int fromload)
 	}
 
 	smdi_loaded = 0;
+
+	/* For Optional API. */
+	ast_module_shutdown_ref(ast_module_info->self);
+
 	return 0;
 }