mingw: emulate write(2) that fails with a EPIPE

mingw: emulate write(2) that fails with a EPIPE

On Windows, when writing to a pipe fails, errno is always
EINVAL. However, Git expects it to be EPIPE.

According to the documentation, there are two cases in which write()
triggers EINVAL: the buffer is NULL, or the length is odd but the mode
is 16-bit Unicode (the broken pipe is not mentioned as possible cause).
Git never sets the file mode to anything but binary, therefore we know
that errno should actually be EPIPE if it is EINVAL and the buffer is
not NULL.

See https://msdn.microsoft.com/en-us/library/1570wh78.aspx for more
details.

This works around t5571.11 failing with v2.6.4 on Windows.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>