avutil: add av_fopen_utf8()

fopen() on windows uses UTF-16, we use UTF-8 everywhere, this
function bridges the gap by using avpriv_open()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/APIchanges

@@ -15,6 +15,9 @@ libavutil:     2012-10-22
 
 API changes, most recent first:
 
+2013-11-04 - xxxxxxx - lavu 52.50.100 - avutil.h
+  Add av_fopen_utf8()
+
 2013-08-xx - xxxxxxx - lavu 52.17.0 - avframe.h
   Add AVFrame.flags and AV_FRAME_FLAG_CORRUPT.