RTSP support for IPTV

Large PR for RTSP support, including basic stuff for RTCP.
This add a new dependency against libcurl for RTSP init.

Feel free to slap me if something seems wrong in the code.

However, I've tested it with valgrind, and it seems memory leak-free for now. I've also checked that performances are nearly the same.

I'd really appreciate that someone tests that the old IPTV behavior is not broken.

<b>IMPORTANT</b>
For now, the "interface" field is used to read the RTSP URL. So if you want to test, you have to put "rtsp://server/stream1" instead of "eth0", and the stream will be detected as RTSP. The "port" field is unused for now, but it could be used as a hint for RTP port.
If you think this "interface" field should not be used, feel free to provide me a good solution. This PR is done to test RTSP support, it is large enough, and I didn't want to make it larger (and with more bugs) by changing the web interface and the configuration.