w32:spawn: New flag GPGRT_PROCESS_STDIO_NUL.
* src/gpg-error.h.in (GPGRT_PROCESS_STDIO_NUL): New. * src/spawn-w32.c (w32_open_null): Add ENABLE_NULL_DEVICE argument. (_gpgrt_process_spawn): Follow the change.
- GnuPG-bug-id: T7723
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>