Also disable spawning on Sandbox.

This isn't a functional change, but it makes our logic more clear,
and catches bugs earlier.

changes/feature22976

@@ -1,5 +1,6 @@
   o Minor features (integration, hardening):
-    - Added a new NoExec option to . When this option is set to 1,
+    - Added a new NoExec option, to prevent Tor from running
+      other programs. When this option is set to 1,
       Tor will never try to run another program, regardless of
       the settings of PortForwardingHelper, ClientTransportPlugin,
       or ServerTransportPlugin. Once NoExec is set, it cannot be

src/or/config.c

@@ -1596,7 +1596,7 @@ options_act(const or_options_t *old_options)
   const int transition_affects_guards =
     old_options && options_transition_affects_guards(old_options, options);
 
-  if (options->NoExec) {
+  if (options->NoExec || options->Sandbox) {
     tor_disable_spawning_background_processes();
   }