Low: controller: use pcmk__new_message() to create direct acks

controld_ack_event_directly() previously used create_request() rather than
pcmknew_reply() to create its reply, probably because the function did not
have the original message that pcmknew_reply() requires as an argument.

However we do want replies to be properly formed. Not all callers have the
original message, so use pcmknew_message() with the same arguments that
pcmknew_reply() would (where available). This changes a couple of attributes
in the created message: PCMKXA_SUBT is now correctly PCMKVALUE_RESPONSE,
and PCMK_XA_REFERENCE is now the placeholder "direct-ack" rather than a new
generated ID.