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!

main.go

@@ -78,7 +78,6 @@ var commands = []struct {
 }{
 	{"serve", cmdServe},
 	{"quickstart", cmdQuickstart},
-	{"restart", cmdRestart},
 	{"stop", cmdStop},
 	{"setaccountpassword", cmdSetaccountpassword},
 	{"setadminpassword", cmdSetadminpassword},
@@ -105,6 +104,7 @@ var commands = []struct {
 	{"config domain add", cmdConfigDomainAdd},
 	{"config domain rm", cmdConfigDomainRemove},
 	{"config describe-sendmail", cmdConfigDescribeSendmail},
+	{"config printservice", cmdConfigPrintservice},
 
 	{"checkupdate", cmdCheckupdate},
 	{"cid", cmdCid},
@@ -530,6 +530,27 @@ needs modifications to make it valid.
 	xcheckf(err, "describing config")
 }
 
+func cmdConfigPrintservice(c *cmd) {
+	c.params = ">mox.service"
+	c.help = `Prints a systemd unit service file for mox.
+
+This is the same file as generated using quickstart. If the systemd service file
+has changed with a newer version of mox, use this command to generate an up to
+date version.
+`
+	if len(c.Parse()) != 0 {
+		c.Usage()
+	}
+
+	pwd, err := os.Getwd()
+	if err != nil {
+		log.Printf("current working directory: %v", err)
+		pwd = "/home/mox"
+	}
+	service := strings.ReplaceAll(moxService, "/home/mox", pwd)
+	fmt.Print(service)
+}
+
 func cmdConfigDomainAdd(c *cmd) {
 	c.params = "domain account [localpart]"
 	c.help = `Adds a new domain to the configuration and reloads the configuration.
@@ -802,42 +823,6 @@ new mail deliveries.
 	fmt.Println("mox stopped")
 }
 
-func cmdRestart(c *cmd) {
-	c.help = `Restart mox after validating the configuration file.
-
-Restart execs the mox binary, which have been updated. Restart returns after
-the restart has finished. If you update the mox binary, keep in mind that the
-validation of the configuration file is done by the old process with the old
-binary. The new binary may report a syntax error. If you update the binary, you
-should use the "config test" command with the new binary to validate the
-configuration file.
-
-Like stop, existing connections get a 3 second period for graceful shutdown.
-`
-	if len(c.Parse()) != 0 {
-		c.Usage()
-	}
-	mustLoadConfig()
-
-	ctl := xctl()
-	ctl.xwrite("restart")
-	line := ctl.xread()
-	if line != "ok" {
-		log.Fatalf("restart failed: %s", line)
-	}
-	// Server is now restarting. It will write ok when it is back online again. If it fails, our connection will be closed.
-	buf := make([]byte, 128)
-	n, err := ctl.conn.Read(buf)
-	if err != nil {
-		log.Fatalf("restart failed: %s", err)
-	}
-	s := strings.TrimSuffix(string(buf[:n]), "\n")
-	if s != "ok" {
-		log.Fatalf("restart failed: %s", s)
-	}
-	fmt.Println("mox restarted")
-}
-
 func cmdSetadminpassword(c *cmd) {
 	c.help = `Set a new admin password, for the web interface.