Nevertheless it accepts "w+b".
When checking the Win32 documentation of fopen()/_wfopen(), it clearly
states:
> In addition to the earlier values, the following characters can be
> appended to mode to specify the translation mode for newline
> characters.

This implementation expects 'b' or 't' to be appended, and therefore
"wb+" ends up with an error, whereas the alias "w+b" works perfectly
while it is exactly the same thing. So let's just have glib "translate"
the mode when it can to have it working transparently the same way on
every platform.