[NTUSER] Fix desktop and window station assignment for csrss

[NTUSER] Fix desktop and window station assignment for csrss
- NtUserSetInformationThread: Stub UserThreadUseActiveDesktop and UserThreadRestoreDesktop
- Properly mark the first thread that enters win32k belonging to csrss. At this point we assume that since gpepCSRSS isn't initialized yet, it probably is the first thread.

[WINSRV] Use NtUserSetInformationThread to set the current desktop when needed
-When csrss needs to use user32 or enter win32k, it first needs to assign the current thread to a desktop.