w32: Add GPGRT_PROCESS_ALLOW_SET_FG for gpgrt_process_spawn.

* src/gpg-error.h.in (GPGRT_PROCESS_ALLOW_SET_FG): New.
* src/spawn-w32.c (_gpgrt_process_spawn): Support the flag.

When GPGME will use gpgrt_process_spawn in future, this flag
will be needed to support GPGME_SPAWN_ALLOW_SET_FG.

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