MEDIUM: acme: allow to wait and restart the task for DNS-01

MEDIUM: acme: allow to wait and restart the task for DNS-01

DNS-01 needs a external process which would register a TXT record on a
DNS provider, using a REST API or something else.

To achieve this, the process should read the dpapi sink and wait for
events. With the DNS-01 challenge, HAProxy will put the task to sleep
before asking the ACME server to achieve the challenge. The task then
need to be woke up, using the command implemented by this patch.

This patch implements the "acme challenge_ready" command which should be
used by the agent once the challenge was configured in order to wake the
task up.

Example:
    echo "@1 acme challenge_ready foobar.pem.rsa domain kikyo" | socat /tmp/master.sock -