cli/show: emit new whole-message crypto status output

This allows MUAs that don't want to think about per-mime-part
cryptographic status to have a simple high-level overview of the
message's cryptographic state.

Sensibly structured encrypted and/or signed messages will work fine
with this.  The only requirement for the simplest encryption + signing
is that the message have all of its encryption and signing protection
(the "cryptographic envelope") in a contiguous set of MIME layers at
the very outside of the message itself.

This is because messages with some subparts signed or encrypted, but
with other subparts with no cryptographic protection is very difficult
to reason about, and even harder for the user to make sense of or work
with.

For further characterization of the Cryptographic Envelope and some of
the usability tradeoffs, see here:

   https://dkg.fifthhorseman.net/blog/e-mail-cryptography.html#cryptographic-envelope

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