kdc: Reinstate publicly accessible configuration structure members

kdc: Reinstate publicly accessible configuration structure members

We add some specific configuration options into
KRB5_KDC_CONFIGURATION_COMMON_ELEMENTS, as otherwise Samba no longer has
any way to access those options, other than through the configuration
file.

This is an adaptation to Heimdal:

commit b82815733598da9ba0807ad4754572276b6ffc06
Author: Luke Howard <lukeh@padl.com>
Date:   Thu Jan 20 09:15:24 2022 +1100

    kdc: add accessor functions for KDC request structure

    Add accessor functions for use by Samba and other plugin developers.
    Documentation is in kdc/kdc-accessors.h.