Check PGP Body tags for additional msgtypes

* src/mapihelp.cpp (change_message_class_ipm_note),
  (change_message_class_ipm_note_secure_cex): Also check
  multipart/alternative and multipart/mixed.

Especially PGP/Inline mails sent by Outlook with the old code
is often times multipart/alternative as Outlook sends this by
default.