Simplify secret key export

* Do not ask for "ASCII Armor" as users do not know what this means.
* Do not ask for passphrase-charset for S/MIME we use utf-8.
* Just use a file dialog with a default name.

This should make the secret key export a bit smoother. Especially
right after the keygen when we ask to save the key for a backup
we do not want to offer confusing options afterward.

• GnuPG-Bug-Id: T4089