Stream live video through network with stbt tv

Launching `stbt tv` with `-s` command line argument streams video
through a TCP socket. Used in conjunction with `stbt control`,
it makes remote configuration of the set-top-boxes under test possible.

The TCP port is read from the new `global.live_stream_port` parameter of
the stb-tester config file, or the hard-coded value 4953 is used if it
isn't specified. 4953 is the default value used by 'tcpserversink'.

The receiving side can watch the video stream with
* vlc:
    `vlc tcp://<host>:<port>`
* gst-launch:
    `gst-launch tcpclientsrc host=<host> port=<port> !
        mpegtsdemux ! decodebin2 ! autovideosink`

The bitrate is preset to 1.5 Mbps that is a good compromise of video
quality and bandwitdh; it might require further tuning later when we
have more usage experience.

The `shell_command || :` statement is used to avoid exiting the script
on error when `set -e` is active, that is, the script exits when any
command exits with a non-zero exit status.