Obtain certificates, and wait for the one that matters

The public shape now works end to end: real ports, Caddy in front, and both
programs that need certificates getting them from the same CA -- Caddy for
the front ends over TLS-ALPN-01, the mail server for its own names over
HTTP-01, which Caddy forwards on port 80.

Proved in the lab against Pebble, with a DNS stub answering every name with
the machine's own address, so no public name or public CA is involved:
twenty checks, ending with IMAPS and submissions presenting a certificate
for the mail host that verifies against the CA, and the webmail sending
sign-in to the server as the first-party client the server registered.

Two things the test found, both of which would have shipped:

- The proxy fronted four of the server's five names. The server puts
  ua-auto-config in its own certificate too, so its challenge was never
  forwarded, one name failed, and the whole order failed with it -- leaving
  the mail ports on a self-signed certificate while everything else looked
  healthy. The list now matches what the server asks for.

- Nothing waited for the certificate. An order that fails is not retried on
  its own and a restart does not start a new one, so the install declared
  itself finished over a self-signed certificate. It now waits, asks again
  every 45 seconds, and reports the issuer -- or says plainly that the
  server will keep trying once the domain resolves here, which is the
  ordinary case on a first install.

--acme-directory and --acme-ca-root are what let a private CA be used: the
root is added to the server image's own bundle and given to Caddy, because
neither sees the other's trust store.

internal/plan/plan.go

@@ -61,6 +61,12 @@ type Options struct {
 	Proxy       string // "caddy", "snippets", "none"
 	Choices     []Choice
 	InstallDeps bool // resolve what is missing rather than refusing over it
+
+	// A private ACME CA, for testing the certificate path without asking a
+	// public CA for certificates for names that are not ours. Empty means
+	// Let's Encrypt, which is what an install should use.
+	ACMEDirectory string
+	ACMECARoot    string
 }
 
 // Shape returns what was chosen for a component.