screen: Disable screen creation when --screen manual is used.

Now that the previous commits add a proper fallback and a way to test
this, enable the second half.

As long as the default screen mode (`--screen auto`) *or* the `rc.lua`
doesn't execute too much code in the global context, this wont break
much.

awesome.c

@@ -912,8 +912,14 @@ main(int argc, char **argv)
     ewmh_init_lua();
 
     /* Parse and run configuration file before adding the screens */
-    if (globalconf.no_auto_screen && !luaA_parserc(&xdg, confpath))
+    if (globalconf.no_auto_screen)
+    {
+        /* Disable automatic screen creation, awful.screen has a fallback */
+        globalconf.ignore_screens = true;
+
+        if(!luaA_parserc(&xdg, confpath))
             fatal("couldn't find any rc file");
+    }
 
     /* init screens information */
     screen_scan();