Restore module loading

The module loading feature got broken a while ago, so restore it, but
have it a bit more explicit this time around.

Reviewed-by: Stephen Henson <steve@openssl.org>

apps/s_server.c

@@ -1438,6 +1438,9 @@ int s_server_main(int argc, char *argv[])
         goto end;
     }
 
+    if (!app_load_modules(NULL))
+        goto end;
+
     if (s_key_file == NULL)
         s_key_file = s_cert_file;