Place the Windows VNC agent socket under SystemTemp, falling back to Windows\Temp where it does not exist

client/vnc/server/agent_peercred_windows.go

@@ -14,9 +14,10 @@ import (
 // rests on three other measures, none of which assume the socket path is
 // secret:
 //
-//   - the socket lives in a dedicated directory (agentSocketDir) created
-//     with a DACL granting only SYSTEM and Administrators, so an
-//     unprivileged local user cannot create or squat a socket there;
+//   - the socket lives in a dedicated directory (agentSocketDirPath) under
+//     %SystemRoot%\SystemTemp, created with a DACL granting only SYSTEM and
+//     Administrators, so an unprivileged local user cannot create or squat a
+//     socket there;
 //   - each spawn uses a cryptographically random socket name, so the path
 //     is unguessable before the agent binds it;
 //   - the daemon publishes the path only after confirming the spawned