Nothing advertises the legacy protocols while they are off (LP-7)

While the switch is off, the answers that tell a mail app where to connect
stop offering what the switch closed, so a new phone or desktop app is not
sent to a port that is shut or a sign-in that will be refused:

- Thunderbird-style autoconfig (/mail/config-v1.1.xml and its other
  paths) and Outlook autodiscover leave out IMAP, POP3 and SMTP
  submission.
- PACC (/.well-known/user-agent-configuration.json) offers JMAP, CalDAV,
  CardDAV and WebDAV, and no IMAP, POP3, SMTP or ManageSieve. The document
  is rendered once per configuration load, so the JMAP-only version is
  rendered beside it and chosen per request; the _ua-auto-config digest in
  the suggested zone follows, since it hashes the same document.
- The suggested zone publishes _imap, _imaps, _pop3, _pop3s, _submission
  and _submissions with target "." -- "not offered", RFC 6186 section 3.4 --
  the spec's decision, rather than dropping them: a client that looks is
  told, and an automatically managed zone replaces the old records instead
  of leaving them behind.
- It also drops the TLSA records for ports 993 and 995. A TLS pin for a
  port the switch has closed advertises a service that is not there.
  Submission's 465 keeps its record: the SMTP lock keeps that port open.

The switch is read per answer, as sign-in reads it, so every node agrees
the moment it turns. Inbound mail, MX records and the JMAP, CalDAV and
CardDAV answers are untouched.

tests/e2e/legacy_protocols.py checks all four on a running server: with the
switch on they offer IMAP, POP3 and SMTP (the control); while it is off
they offer none of them and every legacy SRV name has target "."; and once
it is back on, autoconfig and the zone read as they did before. All checks
pass.

crates/common/src/config/network.rs

@@ -47,6 +47,9 @@ pub struct Network {
 #[derive(Clone)]
 pub struct NetworkInfo {
     pub pacc: Pacc,
+    /// inbuxa: the same document without IMAP, POP3, SMTP and ManageSieve,
+    /// served while legacy protocols are off (legacy-protocols LP-7).
+    pub pacc_jmap_only: Pacc,
     pub mxs: Vec<MailExchanger>,
     pub services: VecMap<ServiceProtocol, Service>,
 }
@@ -320,11 +323,26 @@ impl Network {
             }
         }
 
-        let (prefix, suffix) = serde_json::to_string(&pacc)
-            .unwrap_or_default()
-            .rsplit_once(SPLIT_HERE)
-            .map(|(prefix, suffix)| (prefix.to_string(), suffix.to_string()))
-            .unwrap();
+        let split = |pacc: &Configuration| {
+            serde_json::to_string(pacc)
+                .unwrap_or_default()
+                .rsplit_once(SPLIT_HERE)
+                .map(|(prefix, suffix)| Pacc {
+                    prefix: prefix.to_string(),
+                    suffix: suffix.to_string(),
+                })
+                .unwrap()
+        };
+        // inbuxa: legacy-protocols LP-7
+        let pacc_jmap_only = {
+            let mut pacc = pacc.clone();
+            pacc.protocols.imap = None;
+            pacc.protocols.pop3 = None;
+            pacc.protocols.smtp = None;
+            pacc.protocols.managesieve = None;
+            split(&pacc)
+        };
+        let pacc = split(&pacc);
         let mut network = Network {
             node_id: bp.node_id() as u64,
             server_name: default_hostname.to_string(),
@@ -339,7 +357,8 @@ impl Network {
             info: NetworkInfo {
                 mxs: system.mail_exchangers.into_iter().collect(),
                 services: system.services,
-                pacc: Pacc { prefix, suffix },
+                pacc,
+                pacc_jmap_only,
             },
         };