This makes sure that the internal utf8 path names are handled
properly - the normal file handling functions assume path names
are in the native codepage, which isn't utf8.

This assumes that the tools outside of lavf don't use the mkdir
definition. (The tools don't do the same reading of command line
parameters as wchar either - they probably won't handle all possible
unicode file parameters properly, but at least work more predictably
if no utf8/wchar conversion is involved.)

This is moved further down in os_support.h, since windows.h shouldn't
be included before winsock2.h, while io.h needs to be included before
the manual defines for lseek functions.
Signed-off-by: Martin Storsjö <martin@martin.st>