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/import/curl-util.c

@@ -37,7 +37,7 @@ static int curl_glue_on_io(sd_event_source *s, int fd, uint32_t revents, void *u
 
         translated_fd = PTR_TO_FD(hashmap_get(g->translate_fds, FD_TO_PTR(fd)));
 
-        if ((revents & (EPOLLIN|EPOLLOUT)) == (EPOLLIN|EPOLLOUT))
+        if (FLAGS_SET(revents, EPOLLIN | EPOLLOUT))
                 action = CURL_POLL_INOUT;
         else if (revents & EPOLLIN)
                 action = CURL_POLL_IN;