os: replace select with poll

Select has limitations on the fd values it could accept and silently
breaks when it is reached.

libavformat/os_support.c

@@ -236,7 +236,6 @@ int ff_socket_nonblock(int socket, int enable)
 }
 #endif /* CONFIG_NETWORK */
 
-#if CONFIG_FFSERVER
 #if !HAVE_POLL_H
 int poll(struct pollfd *fds, nfds_t numfds, int timeout)
 {
@@ -305,5 +304,3 @@ int poll(struct pollfd *fds, nfds_t numfds, int timeout)
     return rc;
 }
 #endif /* HAVE_POLL_H */
-#endif /* CONFIG_FFSERVER */
-