Add workaround for opaque signed encrypted data

* src/ribbon-callbacks.cpp (do_reader_action): Only use the
mime handling when there is no "BEGIN PGP MESSAGE" in body.

The sign verify split of the old addon allowed you to first
opaque sign a message, then encrypt it. The mime code now
only detects the encryption as this is a standard thing. The
signature is not verified as it is treated as just another plaintext.