HBAC: Better libhbac debugging
    
    Added support for logging via external log function.
    Log provides information about rules evaluating (HBAC_DBG_INFO level)
    and additionally can describe rules (HBAC_DBG_TRACE level).
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2703
    
    Reviewed-by: Pavel Reichl <preichl@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    Reviewed-by: Michal Židek <mzidek@redhat.com>