tool_cb_wrt: fix writing to Windows null device NUL

- Improve console detection.

Prior to this change WriteConsole could be called to write to a handle
that may not be a console, which would cause an error. This issue is
limited to character devices that are not also consoles such as the null
device NUL.

Bug: https://github.com/curl/curl/issues/3175#issuecomment-439068724
Reported-by: Gisle Vanem