listen/bind in deterministic order for consistent error messages, and warn if quickstart cannot find public ip's

without public ip's, the generated mox config will try to listen on 0.0.0.0 and
::, but because there is already a listener for 127.0.0.1:80 (and possibly
others), a bind for 0.0.0.0:80 will fail. explicit public ip's are needed.

the public http listener is useful for ACME validation over http.

for issue #52

smtpserver/server.go

@@ -19,11 +19,14 @@ import (
 	"net"
 	"os"
 	"runtime/debug"
+	"sort"
 	"strconv"
 	"strings"
 	"sync"
 	"time"
 
+	"golang.org/x/exp/maps"
+
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promauto"
 
@@ -175,7 +178,11 @@ func durationDefault(delay *time.Duration, def time.Duration) time.Duration {
 // Listen initializes network listeners for incoming SMTP connection.
 // The listeners are stored for a later call to Serve.
 func Listen() {
-	for name, listener := range mox.Conf.Static.Listeners {
+	names := maps.Keys(mox.Conf.Static.Listeners)
+	sort.Strings(names)
+	for _, name := range names {
+		listener := mox.Conf.Static.Listeners[name]
+
 		var tlsConfig *tls.Config
 		if listener.TLS != nil {
 			tlsConfig = listener.TLS.Config