wkd: Let --install-key write a template policy file.

* tools/wks-util.c (ensure_policy_file): New.
(wks_cmd_install_key): Call it.

(cherry picked from commit 50cd1a58f3a612704a0056386e1d5cd7cb28d57d)

• GnuPG-bug-id: T4753
• Signed-off-by: Werner Koch <wk@gnupg.org>