libdmsg - Get the encryption operational again

* Currently encrypts/decrypts, but the algorithm is really just a
place-holder for something better. It does not use any openssl
algos beyond basi

libdmsg - Get the encryption operational again

* Currently encrypts/decrypts, but the algorithm is really just a
place-holder for something better. It does not use any openssl
algos beyond basic public key exchange, session key exchange, and
raw aes-256-gcm encryption with a block IV increment to prevent replay
attacks.

* Note that in the final protocol there will be two verifiers embedded
in the dmsg itself, rather than tacked on via the transport. One is
the 32-bit header crc (there is also an aux-data crc), and the second
is a 64-bit verifier that the link-level is intended to replace and check.
The dmsg also has a signature and 24 random bits to mix things up.

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