[NTDLL:CSR][CSRSRV] Start to deprecate CSR functions removed on Windows 2003.

[NTDLL:CSR][CSRSRV] Start to deprecate CSR functions removed on Windows 2003.

The server-side CsrSrvIdentifyAlertableThread and CsrSrvSetPriorityClass
functions are completely removed in Win2k3+, and are since stubbed by
CsrSrvUnusedFunction instead. They however were present up to Windows XP,
albeit with an extremely minimal implementation.

The corresponding client-side CsrIdentifyAlertableThread and CsrSetPriorityClass
now become just stubs that either trivially succeed or fail, respectively.

See https://www.geoffchappell.com/studies/windows/win32/csrsrv/api/srvinit/apidispatch.htm
for more information.

- Fix typo "al*T*ertable" --> "alertable".
- Remove ROS-specific CSRSS_IDENTIFY_ALERTABLE_THREAD that was
  deprecated since ages (at least before 2005)!