rtpproto: Support nonblocking reads
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
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.