SilcUInt16 silc_client_command_call(SilcClient client,
                                        SilcClientConnection conn,
                                        const char *command_line, ...);

    Calls and executes the command indicated by the `command_name'.
    The `command_line' is a string which includes the command's name and
    its arguments separated with whitespaces (' ').  If `command_line'
    is non-NULL then all variable arguments are ignored by default.

    If `command_line' is NULL, then the variable arguments define the
    command's name and its arguments.  The first variable argument must
    be the command name.  The variable argument list must be terminated
    with NULL.

    Returns command identifier for this sent command.  It can be used
    to additionally attach to the command reply using the function
    silc_client_command_pending, if needed.  Returns 0 on error.

    The `command' client operation callback will be called when the
    command is executed to indicate whether or not the command executed
    successfully.

    The `command_reply' client operation callbak will be called when reply
    is received from the server to the command.  Application may also use
    the silc_client_command_pending to attach to the command reply.
    The command identifier for silc_client_command_pending function after
    this function call is conn->cmd_ident, which application may use.

    silc_client_command_call(client, conn, NULL, "PING", "silc.silcnet.org",
                             NULL);
    silc_client_command_call(client, conn, "PING silc.silcnet.org");

    This command executes the commands implemented inside the client
    library.  These commands are designed for command line applications,
    but GUI application may call them too if needed.  Alternatively
    application may override the library and use silc_client_command_send
    function instead.