Create admin socket synchronously before privdrop (yggdrasil-network#1201)

Creating UNIX sockets the listen() goroutine that races against the main
one dropping to an unprivileged user may cause startup failure when
privdrop happens before privileged filesystem access.

Setup or fail in New() and only do listen(2) in listen() to avoid this.

```
# yggdrasil -autoconf -user nobody
2024/11/03 21:15:27 Build name: yggdrasil-go
2024/11/03 21:15:27 Build version: 0.5.9
...
2024/11/03 21:15:27 Admin socket failed to listen: listen unix /var/run/yggdrasil.sock: bind: permission denied
```

Rerun, now the order is flipped:
```
# yggdrasil -autoconf -user nobody
2024/11/03 21:15:34 Build name: yggdrasil-go
2024/11/03 21:15:34 Build version: 0.5.9
[...]
2024/11/03 21:15:34 UNIX admin socket listening on /var/run/yggdrasil.sock
[...]
```

Fixes yggdrasil-network#927.

Author: klemensn

src/admin/admin.go

@@ -83,6 +83,52 @@ func New(c *core.Core, log core.Logger, opts ...SetupOption) (*AdminSocket, erro
 	if a.config.listenaddr == "none" || a.config.listenaddr == "" {
 		return nil, nil
 	}
+
+	listenaddr := string(a.config.listenaddr)
+	u, err := url.Parse(listenaddr)
+	if err == nil {
+		switch strings.ToLower(u.Scheme) {
+		case "unix":
+			if _, err := os.Stat(u.Path); err == nil {
+				a.log.Debugln("Admin socket", u.Path, "already exists, trying to clean up")
+				if _, err := net.DialTimeout("unix", u.Path, time.Second*2); err == nil || err.(net.Error).Timeout() {
+					a.log.Errorln("Admin socket", u.Path, "already exists and is in use by another process")
+					os.Exit(1)
+				} else {
+					if err := os.Remove(u.Path); err == nil {
+						a.log.Debugln(u.Path, "was cleaned up")
+					} else {
+						a.log.Errorln(u.Path, "already exists and was not cleaned up:", err)
+						os.Exit(1)
+					}
+				}
+			}
+			a.listener, err = net.Listen("unix", u.Path)
+			if err == nil {
+				switch u.Path[:1] {
+				case "@": // maybe abstract namespace
+				default:
+					if err := os.Chmod(u.Path, 0660); err != nil {
+						a.log.Warnln("WARNING:", u.Path, "may have unsafe permissions!")
+					}
+				}
+			}
+		case "tcp":
+			a.listener, err = net.Listen("tcp", u.Host)
+		default:
+			a.listener, err = net.Listen("tcp", listenaddr)
+		}
+	} else {
+		a.listener, err = net.Listen("tcp", listenaddr)
+	}
+	if err != nil {
+		a.log.Errorf("Admin socket failed to listen: %v", err)
+		os.Exit(1)
+	}
+	a.log.Infof("%s admin socket listening on %s",
+		strings.ToUpper(a.listener.Addr().Network()),
+		a.listener.Addr().String())
+
 	_ = a.AddHandler("list", "List available commands", []string{}, func(_ json.RawMessage) (interface{}, error) {
 		res := &ListResponse{}
 		for name, handler := range a.handlers {
@@ -233,50 +279,6 @@ func (a *AdminSocket) Stop() error {
 
 // listen is run by start and manages API connections.
 func (a *AdminSocket) listen() {
-	listenaddr := string(a.config.listenaddr)
-	u, err := url.Parse(listenaddr)
-	if err == nil {
-		switch strings.ToLower(u.Scheme) {
-		case "unix":
-			if _, err := os.Stat(u.Path); err == nil {
-				a.log.Debugln("Admin socket", u.Path, "already exists, trying to clean up")
-				if _, err := net.DialTimeout("unix", u.Path, time.Second*2); err == nil || err.(net.Error).Timeout() {
-					a.log.Errorln("Admin socket", u.Path, "already exists and is in use by another process")
-					os.Exit(1)
-				} else {
-					if err := os.Remove(u.Path); err == nil {
-						a.log.Debugln(u.Path, "was cleaned up")
-					} else {
-						a.log.Errorln(u.Path, "already exists and was not cleaned up:", err)
-						os.Exit(1)
-					}
-				}
-			}
-			a.listener, err = net.Listen("unix", u.Path)
-			if err == nil {
-				switch u.Path[:1] {
-				case "@": // maybe abstract namespace
-				default:
-					if err := os.Chmod(u.Path, 0660); err != nil {
-						a.log.Warnln("WARNING:", u.Path, "may have unsafe permissions!")
-					}
-				}
-			}
-		case "tcp":
-			a.listener, err = net.Listen("tcp", u.Host)
-		default:
-			a.listener, err = net.Listen("tcp", listenaddr)
-		}
-	} else {
-		a.listener, err = net.Listen("tcp", listenaddr)
-	}
-	if err != nil {
-		a.log.Errorf("Admin socket failed to listen: %v", err)
-		os.Exit(1)
-	}
-	a.log.Infof("%s admin socket listening on %s",
-		strings.ToUpper(a.listener.Addr().Network()),
-		a.listener.Addr().String())
 	defer a.listener.Close()
 	for {
 		conn, err := a.listener.Accept()