Attempt to read from /dev/urandom and /dev/random with O_NONBLOCK set.
If neither succeeds, proceed with fallbacks.
Originally committed as revision 23903 to svn://svn.ffmpeg.org/ffmpeg/trunk
Keep an old ff_ named function for binary compatibility until the
next major bump.
Originally committed as revision 23254 to svn://svn.ffmpeg.org/ffmpeg/trunk