test536: do not fail with threaded DNS resolver

Also tweaked comments in certain examples using curl_multi_fdset().

docs/examples/multi-debugcallback.c

@@ -140,7 +140,7 @@ int main(int argc, char **argv)
     fd_set fdread;
     fd_set fdwrite;
     fd_set fdexcep;
-    int maxfd;
+    int maxfd = -1;
 
     FD_ZERO(&fdread);
     FD_ZERO(&fdwrite);
@@ -154,8 +154,10 @@ int main(int argc, char **argv)
     curl_multi_fdset(multi_handle, &fdread, &fdwrite, &fdexcep, &maxfd);
 
     /* In a real-world program you OF COURSE check the return code of the
-       function calls, *and* you make sure that maxfd is bigger than -1
-       so that the call to select() below makes sense! */
+       function calls.  On success, the value of maxfd is guaranteed to be
+       greater or equal than -1.  We call select(maxfd + 1, ...), specially in
+       case of (maxfd == -1), we call select(0, ...), which is basically equal
+       to sleep. */
 
     rc = select(maxfd+1, &fdread, &fdwrite, &fdexcep, &timeout);