Add rlm_hmac (similar to rlm_digest), useful for STUN/TURN and possibly DNS TSIG and other


This is an initial cut of rlm_hmac

I've tested it with radclient

We are discussing it in the various open-source TURN server projects

https://groups.google.com/forum/#!topic/turn-server-project-rfc5766-turn-server/A7GefqomAZ0

and would appreciate any feedback from the RADIUS community about how to make this into an official part of RADIUS