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>

net/dns/wsl_windows.go

@@ -228,8 +228,8 @@ func wslRun(cmd *exec.Cmd) (err error) {
 	}
 
 	cmd.SysProcAttr = &syscall.SysProcAttr{
-		Token:      syscall.Token(token),
-		HideWindow: true,
+		CreationFlags: windows.CREATE_NO_WINDOW,
+		Token:         syscall.Token(token),
 	}
 	return cmd.Run()
 }