Add macro for checking if some flags are set

This way we don't need to repeat the argument twice.
I didn't replace all instances. I think it's better to leave out:
- asserts
- comparisons like x & y == x, which are mathematically equivalent, but
  here we aren't checking if flags are set, but if the argument fits in the
  flags.

src/shared/condition.c

@@ -251,7 +251,7 @@ static int condition_test_control_group_controller(Condition *c) {
                 return 1;
         }
 
-        return (system_mask & wanted_mask) == wanted_mask;
+        return FLAGS_SET(system_mask, wanted_mask);
 }
 
 static int condition_test_group(Condition *c) {