mingw_fopen: report ENOENT for invalid file names

mingw_fopen: report ENOENT for invalid file names

On Windows, certain characters are prohibited in file names, most
prominently the colon. When fopen() is called with such an invalid file
name, the underlying Windows API actually reports a particular error,
but since there is no suitable errno value, this error is translated
to EINVAL. Detect the case and report ENOENT instead.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>