ipa-kdb: support KDB DAL version 6.1
    
    DAL version 6.0 removed support for a callback to free principal.
    This broke KDB drivers which had complex e_data structure within
    the principal structure. As result, FreeIPA KDB driver was leaking
    memory with DAL version 6.0 (krb5 1.15).
    
    DAL version 6.1 added a special callback for freeing e_data structure.
    See details at krb5/krb5#596
    
    Restructure KDB driver code to provide this callback in case
    we are built against DAL version that supports it. For DAL version
    prior to 6.0 use this callback in the free_principal callback to
    tidy the code.
    
    Use explicit KDB version dependency in Fedora 26+ via BuildRequires.
    
    With new DAL version, freeipa package will fail to build and
    we'll have to add a support for new DAL version explicitly.
    
    https://fedorahosted.org/freeipa/ticket/6619
    
    Reviewed-By: Simo Sorce <ssorce@redhat.com>
    Reviewed-By: Robbie Harwood <rharwood@redhat.com>