feature request: support for RADIUS authentication

[dpocock]

Support for authentication against a RADIUS server.

For SIP, this can be done using rlm_digest in the FreeRADIUS server. For TURN, it needs to use HMAC instead of DIGEST, so a server-side module in FreeRADIUS server is required.

This is discussed in some detail in the reSIProcate (reTurnServer) wiki:
http://www.resiprocate.org/Improving_RADIUS_Support
http://www.resiprocate.org/RADIUS
https://www.resiprocate.org/bugzilla/show_bug.cgi?id=103

coTurn mailing list:
https://groups.google.com/forum/#!topic/turn-server-project-rfc5766-turn-server/A7GefqomAZ0

[misi]

@dpocock
Sorry I am overloaded.
Could you please send a PR.?