Abort inactive rtsp client connection after timeout

Allow a user to specify the time (in seconds) to wait until the rtsp connection
to a server is considered inactive. After the timeout, the connection will be
terminated with `ETIMEOUT`. By default, the connection timeout is set to 10
seconds. The timeout value is set using the `timeout' command line argument.

Issue: https://trac.ffmpeg.org/ticket/2294

Notes: it is possible that the timeout that occurred is as long as twice the
specified value. The reason is that when UDP is used, the rtsp implementation
will use TCP as a fallback. The fallback connection resets the timeout_cnt, which
will result in the twice the specified value. This issue could be fixed, but it's better
to address that in a seperate commit / PR.