rtsp: Don't include the listen flag in the SDP demuxer flags

It's only relevant for the RTSP demuxer. Similarly, the custom_io
flag is only present in the SDP demuxer options list.
avio: Add an avio callback function for setting nonblocking mode

When opening an avio context based on the built-in urlcontexts,
support setting nonblocking mode.
rtpenc: Support parsing incoming RTCP RR packets

These received data is written as a textual log, accessible via the
"rtcp_log" AVOption. The RTP muxer appends data to this string, and
the caller can clear the string between calls to the muxer if so desired.
avopt: Allow setting string/binary options to NULL

Previously you couldn't set them back to NULL, only to a zero-length
string. Now it's only disallowed to pass NULL when setting numeric
options.
testing only: avconv: Open the output in readwrite mode
testing only: avconv: Print received RTCP log data

Fetching option data with av_opt_get is a bit inefficient since
it always allocates a new buffer for the return value instead
of just returning the pointer.
testing only: rtsp: Send back RTCP RR even in TCP mode

If used like this, the function from rtspenc.c could should be moved
to rtsp.c, otherwise builds with disabled rtsp muxer would fail.

This is only for getting some RTCP RR packets in TCP mode at all
to try to handle.
rtspenc: Add parse_rtcp and rtcp_log options

Ideally, parse_rtcp could be a flag in the rtsp_flags field, but
it's kept as a separate option to be similar to the options in the
RTP muxer.

This handles marshalling of the option into the internal RTP muxers,
and propagating the log back to the RTSP muxer so that the caller
can get it.
rtspenc: Receive RTCP packets over TCP, pass them to the RTP muxer for parsing