Set _O_BINARY as default fmode for both MinGW and MSVC

Set _O_BINARY as default fmode for both MinGW and MSVC

MinGW set the _CRT_fmode to set both the default fmode and _O_BINARY on
stdin/stdout/stderr. Rather use the main() define in mingw.h to set this
for both MinGW and MSVC.

This will ensure that a MinGW and MSVC build will handle input and output
identically.

Signed-off-by: Marius Storm-Olsen <mstormo@gmail.com>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>