Autoselect draft enc key if set to auto

* src/cryptcontroller.cpp (CryptController::resolve_keys): Auto select
draft encryption key if the value is "auto".
* src/main.cpp (write_options): Write draft_key.

This allows administrators to set draft_key under HKLM to
auto and then GpgOL will do draft encryption by selecting
the first ultimately trusted encryption key for which
a secret is available.

• GnuPG-Bug-Id: T5564