krb5: Lookahead resolving of host names
    
    The caller that initializes
    the fail over service (maybe with be_fo_add_service) should provide
    a hint with the value of the lookahead option. Then, if a request for
    server resolution is triggered, the fail over code would resolve a server
    and afterwards check if enough fo_server entries with a valid hostname
    in the struct server_common structure. If not, the request would
    check if any of the fo_server structures represents a SRV query and
    try to resolve the query to receive more host names.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3975
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>