portlist: set SysProcAttr.HideWindow on Windows

Prevents annoying shell window flashes when running /server by hand.

portlist/netstat_exec.go

@@ -13,12 +13,22 @@ import (
 	exec "tailscale.com/tempfork/osexec"
 )
 
+var osHideWindow func(*exec.Cmd) // non-nil on Windows; see portlist_windows.go
+
+// hideWindow returns c. On Windows it first sets SysProcAttr.HideWindow.
+func hideWindow(c *exec.Cmd) *exec.Cmd {
+	if osHideWindow != nil {
+		osHideWindow(c)
+	}
+	return c
+}
+
 func listPortsNetstat(arg string) (List, error) {
 	exe, err := exec.LookPath("netstat")
 	if err != nil {
 		return nil, fmt.Errorf("netstat: lookup: %v", err)
 	}
-	output, err := exec.Command(exe, arg).Output()
+	output, err := hideWindow(exec.Command(exe, arg)).Output()
 	if err != nil {
 		xe, ok := err.(*exec.ExitError)
 		stderr := ""