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!

http/web.go

@@ -9,6 +9,7 @@ import (
 	golog "log"
 	"net"
 	"net/http"
+	"os"
 	"strings"
 	"time"
 
@@ -36,9 +37,9 @@ func safeHeaders(fn http.HandlerFunc) http.HandlerFunc {
 	}
 }
 
-// ListenAndServe starts listeners for HTTP, including those required for ACME to
-// generate TLS certificates.
-func ListenAndServe() {
+// Listen binds to sockets for HTTP listeners, including those required for ACME to
+// generate TLS certificates. It stores the listeners so Serve can start serving them.
+func Listen() {
 	type serve struct {
 		kinds     []string
 		tlsConfig *tls.Config
@@ -179,7 +180,7 @@ func ListenAndServe() {
 
 		for port, srv := range portServe {
 			for _, ip := range l.IPs {
-				listenAndServe(ip, port, srv.tlsConfig, name, srv.kinds, srv.mux)
+				listen1(ip, port, srv.tlsConfig, name, srv.kinds, srv.mux)
 			}
 		}
 	}
@@ -198,7 +199,11 @@ func adminIndex(w http.ResponseWriter, r *http.Request) {
 	http.Redirect(w, r, "/admin/", http.StatusSeeOther)
 }
 
-func listenAndServe(ip string, port int, tlsConfig *tls.Config, name string, kinds []string, mux *http.ServeMux) {
+// functions to be launched in goroutine that will serve on a listener.
+var servers []func()
+
+// listen prepares a listener, and adds it to "servers", to be launched (if not running as root) through Serve.
+func listen1(ip string, port int, tlsConfig *tls.Config, name string, kinds []string, mux *http.ServeMux) {
 	addr := net.JoinHostPort(ip, fmt.Sprintf("%d", port))
 
 	var protocol string
@@ -206,18 +211,23 @@ func listenAndServe(ip string, port int, tlsConfig *tls.Config, name string, kin
 	var err error
 	if tlsConfig == nil {
 		protocol = "http"
-		xlog.Print("http listener", mlog.Field("name", name), mlog.Field("kinds", strings.Join(kinds, ",")), mlog.Field("address", addr))
-		ln, err = net.Listen(mox.Network(ip), addr)
+		if os.Getuid() == 0 {
+			xlog.Print("http listener", mlog.Field("name", name), mlog.Field("kinds", strings.Join(kinds, ",")), mlog.Field("address", addr))
+		}
+		ln, err = mox.Listen(mox.Network(ip), addr)
 		if err != nil {
-			xlog.Fatalx("http: listen"+mox.LinuxSetcapHint(err), err, mlog.Field("addr", addr))
+			xlog.Fatalx("http: listen", err, mlog.Field("addr", addr))
 		}
 	} else {
 		protocol = "https"
-		xlog.Print("https listener", mlog.Field("name", name), mlog.Field("kinds", strings.Join(kinds, ",")), mlog.Field("address", addr))
-		ln, err = tls.Listen(mox.Network(ip), addr, tlsConfig)
-		if err != nil {
-			xlog.Fatalx("https: listen"+mox.LinuxSetcapHint(err), err, mlog.Field("addr", addr))
+		if os.Getuid() == 0 {
+			xlog.Print("https listener", mlog.Field("name", name), mlog.Field("kinds", strings.Join(kinds, ",")), mlog.Field("address", addr))
 		}
+		ln, err = mox.Listen(mox.Network(ip), addr)
+		if err != nil {
+			xlog.Fatalx("https: listen", err, mlog.Field("addr", addr))
+		}
+		ln = tls.NewListener(ln, tlsConfig)
 	}
 
 	server := &http.Server{
@@ -225,8 +235,20 @@ func listenAndServe(ip string, port int, tlsConfig *tls.Config, name string, kin
 		TLSConfig: tlsConfig,
 		ErrorLog:  golog.New(mlog.ErrWriter(xlog.Fields(mlog.Field("pkg", "net/http")), mlog.LevelInfo, protocol+" error"), "", 0),
 	}
-	go func() {
+	serve := func() {
 		err := server.Serve(ln)
 		xlog.Fatalx(protocol+": serve", err)
-	}()
+	}
+	servers = append(servers, serve)
+}
+
+// Serve starts serving on the initialized listeners.
+func Serve() {
+	go manageAuthCache()
+	go importManage()
+
+	for _, serve := range servers {
+		go serve()
+	}
+	servers = nil
 }