[WINLOGON] Clean up part 1

- Add empty lines to separate logical blocks of code.
- Add spaces according to our coding style.
- Move g_hShutdownDialog and g_bShuttingDown into the shutdown parameters struct.
- Pass a pointer to the shutdown parameters to the shutdown thread and the shutdown dialog and use the pointer instead of the global variables.
- Move the timer code into a separate function because it requires local variables.