Wait for admin goroutine to create control socket before changing user

Otherwise the main goroutine races dropping privileges races against
those carrying out privileges tasks at startup.

Reproducible at least on OpenBSD/amd64 7.6-current, where it (expectedly)
fails to create a UNIX socket in `0755 root:wheel /var/run/` *after*
calling setuid(2) to `nobody`:
```
# 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
[...]
```

The `AdminSocket`s `done` channel is insufficient to sync here, waiting
for `n.admin.IsStarted()` does not guarantee socket creation, so export
a simple boolean instead.

This is a minimal fix to prevent startup failure;  TUN interface and
raw socket creation (to manage routes) might suffer the same problem,
but I have not seem them fail yet.

Fixes yggdrasil-network#927.

Author: klemensn

cmd/yggdrasil/main.go

@@ -281,6 +281,14 @@ func main() {
 		<-done
 	})
 
+	// Wait for other goroutines to finish potentially privileged tasks before dropping privileges.
+	for {
+		// control socket:  UNIX requires filesystem permissions, TCP may use a low privileged port.
+		if n.admin.Created {
+			break
+		}
+	}
+
 	// Change user if requested
 	if *chuserto != "" {
 		err = chuser(*chuserto)

src/admin/admin.go

@@ -26,6 +26,7 @@ type AdminSocket struct {
 	config   struct {
 		listenaddr ListenAddress
 	}
+	Created  bool
 }
 
 type AdminSocketRequest struct {
@@ -274,6 +275,7 @@ func (a *AdminSocket) listen() {
 		a.log.Errorf("Admin socket failed to listen: %v", err)
 		os.Exit(1)
 	}
+	a.Created = true
 	a.log.Infof("%s admin socket listening on %s",
 		strings.ToUpper(a.listener.Addr().Network()),
 		a.listener.Addr().String())