Handle unicode file names on windows

All file names should be in UTF-8 within libavformat.

This is handled by mapping the open() function to an internal one
in os_support.h for windows.

fopen() could be overridden in the same way, but if that would be
used from ffmpeg.c, it would add a dependency on an ff prefixed
internal lavf function.

Signed-off-by: Martin Storsjö <martin@martin.st>