child.c: properly initialize fdset for each select() call (#216)

it was reported that because the fdset was only initialized once,
tinyproxy would fail to properly listen on more than one interface.

closes #214
closes #127

src/child.c

@@ -206,6 +206,9 @@ static void child_main (struct child_s *ptr)
          * We have to wait for connections on multiple fds,
          * so use select.
          */
+        while (!config.quit) {
+
+                int listenfd = -1;
 
                 FD_ZERO(&rfds);
 
@@ -224,9 +227,6 @@ static void child_main (struct child_s *ptr)
                         maxfd = max(maxfd, *fd);
                 }
 
-        while (!config.quit) {
-                int listenfd = -1;
-
                 ptr->status = T_WAITING;
 
                 clilen = sizeof(struct sockaddr_storage);