Preselect sign / encrypt for reply / forward
* src/mail.cpp (Mail::Mail): Initalize crypto flags. (Mail::get_crypto_flags): New. (Mail::parsing_done): Set crypto flags. * src/mailitem-events.cpp (EVENT_SINK_INVOKE): Handle reply, forward and ReplyAll.
This sets sign for signed mails, encrypt for encrypted mails
and sign & encrypt for signed and encrypted mails.