spawn: Add new GPGRT_PROCESS_NO_CONSOLE for Windows.

* src/gpg-error.h.in (GPGRT_PROCESS_NO_CONSOLE): New.
* src/spawn-w32.c (_gpgrt_process_spawn): Handle
GPGRT_PROCESS_NO_CONSOLE.

This may be used to support ASSUAN_PIPE_CONNECT_DETACHED
in libassuan.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>