Sort password policies properly with --pkey-only
    
    Password policy plugin sorts password policies by its COS priority.
    However, when the pwpolicy-find command is run with --pkey-only,
    the resulting entries do not contain COS priority and the sort
    function crashes.
    
    This patch makes sure that cospriority is present in the time
    of the result sorting process and removes the cospriority again
    when the sorting is done. This way, the entries are sorted properly
    both with and without --pkey-only flag.
    
    Previous entries_sortfn member attribute of LDAPSearch class
    containing custom user sorting function was replaced just with
    a flag indicating if a sorting in LDAPSearch shall be done at all.
    This change makes it possible to sort entries in a custom
    post_callback which is much more powerful (and essential for
    sorting like in pwpolicy plugin) approach than a plain sorting
    function.
    
    https://fedorahosted.org/freeipa/ticket/2676