Implement MIT Kerberos localauth plugin
    
    The MIT Kerberos localauth pluing interface defines two different calls.
    
    The first checks if a given Kerberos principal relates to a given name
    of a local user (userok). The implementation lets SSSD resolve the
    principal and the user name and if the returned user entries both have
    the same UID success is returned.
    
    The second translates a given Kerberos principal to a local user name
    (a2l). Here SSSD is only called once to resolve the principal and the
    user name is returned.
    
    Resolves https://fedorahosted.org/sssd/ticket/1835
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>