Offer encryption with lower security also for file encryption

If S/MIME encryption fails because some recipients are invalid then we
offer to retry the operation without thorough validation of the S/MIME
certificates. (For the Notepad we already do the same.)

(cherry picked from commit 9d5f9d949e5181c4c77f3ab52053de2c09baf0ef)

• GnuPG-bug-id: T6702