Mention old preauth header file in docs

The previous commit updated the header file references for 1.12 in the
clpreauth and kdcpreauth plugin interface documentation.  Add a
parenthetical so that the reference is still useful for prior
releases.
Add libkrad

The new library libkrad provides code for the parsing of RADIUS packets
as well as client implementation based around libverto.  This library
should be considered unstable.

ticket: 7678 (new)
Add server-side otp preauth plugin

This plugin implements the proposal for providing OTP support by
proxying requests to RADIUS. Details can be found inside the
provided documentation as well as on the project page.

http://k5wiki.kerberos.org/wiki/Projects/OTPOverRADIUS

ticket: 7678