Add support for negotiating IMAP and SMTP on the HTTPS port 443 using TLS ALPN "imap" and "smtp"

Intended for future use with chatmail servers. Standard email ports may be
blocked on some networks, while the HTTPS port may be accessible.

This is a squashed commit of PR #255 by s0ph0s-dog.

docker-compose-integration.yml

@@ -26,6 +26,8 @@ services:
         condition: service_healthy
       localserve:
         condition: service_healthy
+      moxacmepebblealpn:
+        condition: service_healthy
     networks:
       mailnet1:
         ipv4_address: 172.28.1.50
@@ -83,6 +85,31 @@ services:
       mailnet1:
         ipv4_address: 172.28.1.20
 
+  # Third mox instance that uses ACME with pebble and has ALPN enabled.
+  moxacmepebblealpn:
+    hostname: moxacmepebblealpn.mox1.example
+    domainname: mox1.example
+    image: mox_integration_moxmail
+    environment:
+      MOX_UID: "${MOX_UID}"
+    command: ["sh", "-c", "/integration/moxacmepebblealpn.sh"]
+    volumes:
+      - ./testdata/integration/resolv.conf:/etc/resolv.conf
+      - ./testdata/integration:/integration
+    healthcheck:
+      test: netstat -nlt | grep ':25 '
+      interval: 1s
+      timeout: 1s
+      retries: 10
+    depends_on:
+      dns:
+        condition: service_healthy
+      acmepebble:
+        condition: service_healthy
+    networks:
+      mailnet1:
+        ipv4_address: 172.28.1.80
+
   localserve:
     hostname: localserve.mox1.example
     domainname: mox1.example