Start mail handing restructuring
* src/mail.cpp (Mail::process_message): Split out beforeRead parts into pre_process_message. (get_cipherstream): New helper to get OOM level attachment. (Mail::decrypt_mime): New. OOM based mime decryption. (Mail::decrypt): Dummy. (Mail::do_crypto): Rename to encrypt_sign. (Mail::decrypt_verify): New. Entry point for Read event. * src/mail.h: Update accordingly. * src/mailitem-events.cpp: Call new functions.
First part of a series of commits that will rework the way
decryption works without modifing the underlying mapi object.