Add hostrealm pluggable interface definition

ticket: 7687 (new)
Use hostrealm interface for realm mapping

Reimplement krb5_get_host_realm, krb5_get_fallback_host_realm, and
krb5_get_default_realm in terms of the hostrealm interface.  Three
built-in modules (dns, domain, and profile) implement the current
behavior.

ticket: 7687
Move utility functions to hostrealm.c

Move the remaining internal functions from hst_realm.c to hostrealm.c,
and get rid of hst_realm.c.

ticket: 7687
Add hostrealm interface tests

Create a test module for the hostrealm interface, a harness to call
the realm mapping functions and display their results, and a Python
script to exercise the functionality of the interface and each module
(except the dns module, which we cannot easily test since it relies on
TXT records in the public DNS).

ticket: 7687
Document hostrealm interface

ticket: 7687