LDAP: Add sdap_lookup_type enum
    
    Related:
        https://fedorahosted.org/sssd/ticket/2553
    
    Change the boolan parameter of sdap_get_users_send and sdap_get_groups_send
    to a tri-state that controls whether we expect only a single entry
    (ie don't use the paging control), multiple entries with a search limit
    (wildcard request) or multiple entries with no limit (enumeration).
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>