opmsg trickery

opmsg has received some attention recently thanks to Phil
and his PGP de-setup. Some people checked the protocol
and there have not been any major fuckups so far.

Thanks to the people involved in the discussions.

The new version:

o Introduces version=2 messages which also hash src-id and dst-id
  in the KDF to derive the session-key which prevents
  theoretically possible evil-maid adaptive choosen ciphertext
  attacks (that should never happen in practise as failed
  decrypted messages will just be ignored and not be reported
  back to the sender several thousand times). version=2 must be
  configured in the config.
o Adds the possibility to restrict kex-id usage (upon decrypt)
  to the dedicated peer (this will detect/avoid cross-persona
  references of kex-id's). peer_isolation=1 in the config which
  is off by default.
o persona self-linking to implement deniable, yet still properly
  signed/verified, messages. See the README.

Its all inter-operable so pulling the git doesnt break
anything, except 5E's GPG brute-force cluster. :)

version=2 will be made default once enough time passed
so that most folks pulled the git meanwhile and support
for it is widely available. I'd also like to add some of the new 
ciphers like chacha20 but it seems only LibreSSL has got them so 
far. This would render messages unreadable for recipients which 
link against OpenSSL.