KDC Audit infrastructure and plugin implementation

Per project http://k5wiki.kerberos.org/wiki/Projects/Audit

The purpose of this project is to create an Audit infrastructure to monitor
security related events on the KDC.

The following events are targeted in the initial version:
- startup and shutdown of the KDC;
- AS_REQ and TGS_REQ exchanges.  This includes client address and port, KDC
  request and request ID, KDC reply, primary and derived ticket and their
  ticket IDs, second ticket ID, cross-realm referral, was ticket renewed and
  validated, local policy violation and protocol constraints, and KDC status
  message.

Ticket ID is introduced to allow to link tickets to their initial TGT at any
stage of the Kerberos exchange. For the purpose of this project it is a private
to KDC ticket ID: each successfully created ticket is hashed and recorded
into audit log. The administrators can correlate the primary and derived
ticket IDs after the fact.

Request ID is a randomly generated alpha-numeric string. Using this ID an
administrator can easily correlate multiple audit events related to a single
request. It should be informative both in cases when the request is sent to
multiple KDCs, or to the same KDC multiple times.

For the purpose of testing and demo of the Audit, the JSON based modules are
implemented: "test" and "simple" audit modules respectively.
The file plugins/audit/j_dict.h is a dictionary used in this implememtations.

The new Audit system is build-time enabled and run-time pluggable.

[kaduk@mit.edu: remove potential KDC crashes, minor reordering]

ticket: 7712
target_version: 1.12