MEDIUM: acme: better error/retry management of the challenge checks

MEDIUM: acme: better error/retry management of the challenge checks

When the ACME task is checking for the status of the challenge, it would
only succeed or retry upon failure.

However that's not the best way to do it, ACME objects contain an
"status" field which could have a final status or a in progress status,
so we need to be able to retry.

This patch adds an acme_ret enum which contains OK, RETRY and FAIL.

In the case of the CHKCHALLENGE, the ACME could return a "pending" or a
"processing" status, which basically need to be rechecked later with the
RETRY. However a "invalid" or "valid" status is final and will return
either a FAIL or a OK.

So instead of retrying in any case, the "invalid" status will ends the
task with an error.