* src/common_indep.h (msgtype_t): New messagtype for WKS_Confirm
mails.
* src/mail.cpp (Mail::decrypt_verify): Handle new message type.
(Mail::is_smime): Clarify an error message.
* src/mapihelp.cpp
(change_message_class_ipm_note_smime_multipartsigned),
(string_to_type, mapi_change_message_class),
(mapi_get_message_content_type): Handle
wks confirmation mails.
* src/message.cpp: Default / ignore new message type in old code.
* src/wks-helper.cpp: Lots of changes.