w32: Change spawn functions to use Unicode version of CreateProcess.

* common/exechelp-w32.c (gnupg_spawn_process): Change to use
CreateProcessW.
(gnupg_spawn_process_fd): Ditto.
(gnupg_spawn_process_detached): Ditto.
* g10/exec.c (w32_system): Ditto.

We do not use this weirdo CREATE_UNICODE_ENVIRONMENT flag because it
does not make any sense to have non-ASCII names in the environment. I
can't imagine why this should be used at all and rationale for this
API features is, well, sparse.

• GnuPG-bug-id: T4398