change mox to start as root, bind to network sockets, then drop to regular unprivileged mox user

makes it easier to run on bsd's, where you cannot (easily?) let non-root users
bind to ports <1024. starting as root also paves the way for future improvements
with privilege separation.

unfortunately, this requires changes to how you start mox. though mox will help
by automatically fix up dir/file permissions/ownership.

if you start mox from the systemd unit file, you should update it so it starts
as root and adds a few additional capabilities:

        # first update the mox binary, then, as root:
        ./mox config printservice >mox.service
        systemctl daemon-reload
        systemctl restart mox
        journalctl -f -u mox &
        # you should see mox start up, with messages about fixing permissions on dirs/files.

if you used the recommended config/ and data/ directory, in a directory just for
mox, and with the mox user called "mox", this should be enough.

if you don't want mox to modify dir/file permissions, set "NoFixPermissions:
true" in mox.conf.

if you named the mox user something else than mox, e.g. "_mox", add "User: _mox"
to mox.conf.

if you created a shared service user as originally suggested, you may want to
get rid of that as it is no longer useful and may get in the way. e.g. if you
had /home/service/mox with a "service" user, that service user can no longer
access any files: only mox and root can.

this also adds scripts for building mox docker images for alpine-supported
platforms.

the "restart" subcommand has been removed. it wasn't all that useful and got in
the way.

and another change: when adding a domain while mtasts isn't enabled, don't add
the per-domain mtasts config, as it would cause failure to add the domain.

based on report from setting up mox on openbsd from mteege.
and based on issue #3. thanks for the feedback!

smtpserver/server.go

@@ -161,8 +161,9 @@ var (
 
 var jitterRand = mox.NewRand()
 
-// ListenAndServe starts network listeners that serve incoming SMTP connection.
-func ListenAndServe() {
+// 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 {
 		var tlsConfig *tls.Config
 		if listener.TLS != nil {
@@ -181,7 +182,7 @@ func ListenAndServe() {
 			}
 			port := config.Port(listener.SMTP.Port, 25)
 			for _, ip := range listener.IPs {
-				go listenServe("smtp", name, ip, port, hostname, tlsConfig, false, false, maxMsgSize, false, listener.SMTP.RequireSTARTTLS, listener.SMTP.DNSBLZones)
+				listen1("smtp", name, ip, port, hostname, tlsConfig, false, false, maxMsgSize, false, listener.SMTP.RequireSTARTTLS, listener.SMTP.DNSBLZones)
 			}
 		}
 		if listener.Submission.Enabled {
@@ -191,7 +192,7 @@ func ListenAndServe() {
 			}
 			port := config.Port(listener.Submission.Port, 587)
 			for _, ip := range listener.IPs {
-				go listenServe("submission", name, ip, port, hostname, tlsConfig, true, false, maxMsgSize, !listener.Submission.NoRequireSTARTTLS, !listener.Submission.NoRequireSTARTTLS, nil)
+				listen1("submission", name, ip, port, hostname, tlsConfig, true, false, maxMsgSize, !listener.Submission.NoRequireSTARTTLS, !listener.Submission.NoRequireSTARTTLS, nil)
 			}
 		}
 
@@ -202,34 +203,47 @@ func ListenAndServe() {
 			}
 			port := config.Port(listener.Submissions.Port, 465)
 			for _, ip := range listener.IPs {
-				go listenServe("submissions", name, ip, port, hostname, tlsConfig, true, true, maxMsgSize, true, true, nil)
+				listen1("submissions", name, ip, port, hostname, tlsConfig, true, true, maxMsgSize, true, true, nil)
 			}
 		}
 	}
 }
 
-func listenServe(protocol, name, ip string, port int, hostname dns.Domain, tlsConfig *tls.Config, submission, xtls bool, maxMessageSize int64, requireTLSForAuth, requireTLSForDelivery bool, dnsBLs []dns.Domain) {
+var servers []func()
+
+func listen1(protocol, name, ip string, port int, hostname dns.Domain, tlsConfig *tls.Config, submission, xtls bool, maxMessageSize int64, requireTLSForAuth, requireTLSForDelivery bool, dnsBLs []dns.Domain) {
 	addr := net.JoinHostPort(ip, fmt.Sprintf("%d", port))
-	xlog.Print("listening for smtp", mlog.Field("listener", name), mlog.Field("address", addr), mlog.Field("protocol", protocol))
+	if os.Getuid() == 0 {
+		xlog.Print("listening for smtp", mlog.Field("listener", name), mlog.Field("address", addr), mlog.Field("protocol", protocol))
+	}
 	network := mox.Network(ip)
-	var ln net.Listener
-	var err error
-	if xtls {
-		ln, err = tls.Listen(network, addr, tlsConfig)
-	} else {
-		ln, err = net.Listen(network, addr)
-	}
+	ln, err := mox.Listen(network, addr)
 	if err != nil {
-		xlog.Fatalx("smtp: listen for smtp"+mox.LinuxSetcapHint(err), err, mlog.Field("protocol", protocol), mlog.Field("listener", name))
+		xlog.Fatalx("smtp: listen for smtp", err, mlog.Field("protocol", protocol), mlog.Field("listener", name))
 	}
-	for {
-		conn, err := ln.Accept()
-		if err != nil {
-			xlog.Infox("smtp: accept", err, mlog.Field("protocol", protocol), mlog.Field("listener", name))
-			continue
+	if xtls {
+		ln = tls.NewListener(ln, tlsConfig)
+	}
+
+	serve := func() {
+		for {
+			conn, err := ln.Accept()
+			if err != nil {
+				xlog.Infox("smtp: accept", err, mlog.Field("protocol", protocol), mlog.Field("listener", name))
+				continue
+			}
+			resolver := dns.StrictResolver{} // By leaving Pkg empty, it'll be set by each package that uses the resolver, e.g. spf/dkim/dmarc.
+			go serve(name, mox.Cid(), hostname, tlsConfig, conn, resolver, submission, xtls, maxMessageSize, requireTLSForAuth, requireTLSForDelivery, dnsBLs)
 		}
-		resolver := dns.StrictResolver{} // By leaving Pkg empty, it'll be set by each package that uses the resolver, e.g. spf/dkim/dmarc.
-		go serve(name, mox.Cid(), hostname, tlsConfig, conn, resolver, submission, xtls, maxMessageSize, requireTLSForAuth, requireTLSForDelivery, dnsBLs)
+	}
+
+	servers = append(servers, serve)
+}
+
+// Serve starts serving on all listeners, launching a goroutine per listener.
+func Serve() {
+	for _, serve := range servers {
+		go serve()
 	}
 }