[WINLOGON] Usability enhancement for the RPC SystemShutdown functionality.

[WINLOGON] Usability enhancement for the RPC SystemShutdown functionality.

- Perform an immediate system shutdown if the timeout is zero, otherwise
  display the countdown shutdown dialog.
- Cleanup pShutdownParams->pszMessage as soon as the dialog goes away.
- It actually appears (tested on Windows 2000 and 2003) that sending
  WM_CLOSE messages from a user-mode app to the shutdown dialog really
  do nothing (and in particular does not cancel the shutdown!), so modify
  the code to take this fact into account.