Add xrealmauthz KDC policy module and tests

Add xrealmauthz KDC policy module and tests

This module provides fine-grained access control for cross-realm
authentications by checking string attributes on the incoming
cross-realm TGT entry.  It supports realm-based and principal-specific
authorization rules.

The module is not installed by the build system or loaded by default,
and is documented only in the module source code.

[ghudson@mit.edu: simplified code and tests; edited commit message]