proc-cmdline: return proper errors from shall_restore_state()

Let's not eat up errors in shall_restore_state(), but in the consumers
instead, just for the sake of keeping the library calls generic.

src/rfkill/rfkill.c

@@ -212,7 +212,7 @@ static int load_state(
         assert(udev);
         assert(event);
 
-        if (!shall_restore_state())
+        if (shall_restore_state() == 0)
                 return 0;
 
         r = find_device(udev, event, &device);