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/basic/process-util.c

@@ -1359,7 +1359,7 @@ int safe_fork_full(
                 }
         }
 
-        if ((flags & (FORK_NEW_MOUNTNS|FORK_MOUNTNS_SLAVE)) == (FORK_NEW_MOUNTNS|FORK_MOUNTNS_SLAVE)) {
+        if (FLAGS_SET(flags, FORK_NEW_MOUNTNS | FORK_MOUNTNS_SLAVE)) {
 
                 /* Optionally, make sure we never propagate mounts to the host. */