stdin patch by (Charles Yates <charles dot yates at pandora dot be>)

* removes use of read_key and getchar when input is received on stdin
(this was corrupting the packet reading)
* terminates av_encode when a termination signal is received (use of
ctrl-c issued a term_exit and subsequent uses of q failed)
* specific correction to yuv4mpeg pipe reading - the defined header was
too short to allow for extended yuv4mpeg flags [as used by smil2yuv and
y4mscaler and accepted by mjpeg tools]

Originally committed as revision 2183 to svn://svn.ffmpeg.org/ffmpeg/trunk