Add proper Attachment handling for MIME messages.

Attachments are now session decrypted in the read event and
session encrypted again in the write event.

• src/attachment.cpp, src/attachment.h: Functions for attachment

Session encryption / decryption.

• src/Makefile.am: Add new files.
• src/mailitem-events.cpp (~MailItemEvents): Resolve Macro

to avoid confusing bracing.
(MailItemEvents::handle_read): Remove session encryption from
attachments.
(MailItemEvents::invoke): Track "wipe" status. Session encrypt
attachments in write event.

• src/mapihelp.cpp (get_gpgolattachtype): Unstatic.
• src/mapihelp.h (get_gpgolattachtype): Expose.

(attachtype_t): Define attachment type for temporary unprotected
Attachments.

The code will probably be changed to just untprotect an attachment
when it is accessed in the BeforeAttachmentWriteToTempFile event.