cmd/tailscaled, net/dns, wgengine/router: start Windows child processes with DETACHED_PROCESS when I/O is being piped

When we're starting child processes on Windows that are CLI programs that
don't need to output to a console, we should pass in DETACHED_PROCESS as a
CreationFlag on SysProcAttr. This prevents the OS from even creating a console
for the child (and paying the associated time/space penalty for new conhost
processes). This is more efficient than letting the OS create the console
window and then subsequently trying to hide it, which we were doing at a few
callsites.

Fixes #12270

Signed-off-by: Aaron Klotz <aaron@tailscale.com>

cmd/tailscaled/tailscaled_windows.go

@@ -435,6 +435,9 @@ func babysitProc(ctx context.Context, args []string, logf logger.Logf) {
 		startTime := time.Now()
 		log.Printf("exec: %#v %v", executable, args)
 		cmd := exec.Command(executable, args...)
+		cmd.SysProcAttr = &syscall.SysProcAttr{
+			CreationFlags: windows.DETACHED_PROCESS,
+		}
 
 		// Create a pipe object to use as the subproc's stdin.
 		// When the writer goes away, the reader gets EOF.