Add support for S/MIME always trust

* src/cryptcontroller.cpp (prepare_crypto): Split preperation out of.
(do_crypto): Add force parameter improve error handling.
(start_crypto_overlay, stop_crypto_overlay): Make public.
* src/mail.cpp (do_crypt): Handle repeating a crypto operation with
  force.

This allows users to individually override errors from GPGSM
by using the new --always-trust feature of GPGSM. While this
is not optimal this is often times preferrable over not
beeing able to encrypt a mail at all.

• GnuPG-Bug-Id: T6701