cmake: use FindSodium module
cmake: correct find function
Stubbed in gssapi security mechanism.
stubbed in TOKEN command
Split up gssapi mechanism into client and server.
Refactored common impl into gssapi_mechanism_base.

E.g., both client and server need to produce and process GSSAPI tokens.
Added gss produce/process routines.

For exchanging tokens.
Establishing GSSAPI sec context is working now
Methods to send/recv gssapi-wrapped messages
GSSAPI mechanism now fully working with encryption
Added comments to methods.
added ready for meta data exchange at end of gss handshake
:fixed  client state transition bug
gss encode/decode 'more' flag
gss mech sends principle as part of zap request
configurable principle and service principle (for client)
fixed up comments from pr
principle->principal
encode and decode ready message
ZMQ_GSSAPI_PLAINTEXT option for disabling encryption