MINOR: acme/cli: 'acme status' show the status acme-configured certificates

MINOR: acme/cli: 'acme status' show the status acme-configured certificates

The "acme status" command, shows the status of every certificates
configured with ACME, not only the running task like "acme ps".

The IO handler loops on the ckch_store tree and outputs a line for each
ckch_store which has an acme section set. This is still done under the
ckch_store lock and doesn't support resuming when the buffer is full,
but we need to change that in the future.