dbus_message_append_args() adds a reference to memory that is not
copied to the outgoing message until dbus_connection_send() is
called. Since we compile our reply messages in functions and then
return the reply, we need a mechanism for deleting allocated
memory after invoking dbus_connection_send. I have changed the
arguments to sbus_msg_handler_fn so that it takes a talloc ctx
containing the sbus_message_handler_ctx and a pointer to a reply
object. We can now allocate memory as a child of the reply context
and free it after calling dbus_connection_send.