ipn/ipnauth: implement WindowsActor

WindowsActor is an ipnauth.Actor implementation that represents a logged-in
Windows user by wrapping their Windows user token.

Updates #14823

Signed-off-by: Nick Khyl <nickk@tailscale.com>

ipn/ipnauth/ipnauth_windows.go

@@ -36,6 +36,12 @@ type token struct {
 	t windows.Token
 }
 
+func newToken(t windows.Token) *token {
+	tok := &token{t: t}
+	runtime.SetFinalizer(tok, func(t *token) { t.Close() })
+	return tok
+}
+
 func (t *token) UID() (ipn.WindowsUserID, error) {
 	sid, err := t.uid()
 	if err != nil {
@@ -184,7 +190,5 @@ func (ci *ConnIdentity) WindowsToken() (WindowsToken, error) {
 		return nil, err
 	}
 
-	result := &token{t: windows.Token(h)}
-	runtime.SetFinalizer(result, func(t *token) { t.Close() })
-	return result, nil
+	return newToken(windows.Token(h)), nil
 }