Do not use _wfopen() for DOS device filenames

It appears that CONIN$ can be opened correctly by fopen(), but not
_wfopen() (with "CONIN$" passed in the correct format, of course).

Reported by Ray Kolbe.

It was also reported in https://github.com/msysgit/msysgit/issues/153, for
some definition of "reported": it was not only reported to the wrong
repository but also so incomplete as to be contemptous of the time of the
people asked for help.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>