Fix S/MIME sign+encrypt

Summary:
Kleopatra now correctly produces a an encrypted file (.s7m) and a detached
signature (.s7p).

Reviewers: JochenSaalfeld

Reviewed By: JochenSaalfeld

Subscribers: kde-pim, KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D8368