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

http/web.go

@@ -18,6 +18,8 @@ import (
 
 	_ "net/http/pprof"
 
+	"golang.org/x/exp/maps"
+
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promauto"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
@@ -362,7 +364,13 @@ func Listen() {
 		}
 	}
 
-	for name, l := range mox.Conf.Static.Listeners {
+	// Initialize listeners in deterministic order for the same potential error
+	// messages.
+	names := maps.Keys(mox.Conf.Static.Listeners)
+	sort.Strings(names)
+	for _, name := range names {
+		l := mox.Conf.Static.Listeners[name]
+
 		portServe := map[int]*serve{}
 
 		var ensureServe func(https bool, port int, kind string) *serve
@@ -546,7 +554,10 @@ func Listen() {
 			ensureManagerHosts[m] = hosts
 		}
 
-		for port, srv := range portServe {
+		ports := maps.Keys(portServe)
+		sort.Ints(ports)
+		for _, port := range ports {
+			srv := portServe[port]
 			sort.Slice(srv.PathHandlers, func(i, j int) bool {
 				a := srv.PathHandlers[i].Path
 				b := srv.PathHandlers[j].Path

imapserver/server.go

@@ -57,11 +57,12 @@ import (
 	"strings"
 	"time"
 
-	"github.com/prometheus/client_golang/prometheus"
-	"github.com/prometheus/client_golang/prometheus/promauto"
 	"golang.org/x/exp/maps"
 	"golang.org/x/exp/slices"
 
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+
 	"github.com/mjl-/bstore"
 
 	"github.com/mjl-/mox/config"
@@ -307,7 +308,11 @@ type msgseq uint32
 
 // Listen initializes all imap listeners for the configuration, and stores them for Serve to start them.
 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

quickstart.go

@@ -457,6 +457,17 @@ listed in more DNS block lists, visit:
 			fmt.Printf(" OK\n")
 		}
 	}
+
+	if len(publicIPs) == 0 {
+		log.Printf(`WARNING: Could not find your public IP address(es). The "public" listener is
+configured to listen on 0.0.0.0 (IPv4) and :: (IPv6). If you don't change these
+to your actual public IP addresses, you will likely get "address in use" errors
+when starting mox because the "internal" listener binds to a specific IP
+address on the same port(s).
+
+`)
+	}
+
 	fmt.Printf("\n")
 
 	user := "mox"

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