* src/mail.cpp (Mail::Mail): Add new marker for forwarded
crypto mails.
(Mail::remove_our_attachments): New. Removes gpgol attachments.
(Mail::set_is_forwarded_crypto_mail, Mail::is_forwarded_crypto_mail):
New accessors to marker.
* src/mailitem-events.cpp (EVENT_SINK_INVOKE): Set marker on
forward. Handle marker on write.