[CSRSRV] Minimally-adapt CSRSRV to make it able to host other subsystems than Win32.

[CSRSRV] Minimally-adapt CSRSRV to make it able to host other subsystems than Win32.

This is really ReactOS-specific, so I surround them with __REACTOS__
(even if this is our code) to differentiate these from Win2k3 behaviour,
even though they were strongly inspired by what was possible in the beta
versions of NT 3.1 (pre-3.10.404).
Interestingly, Windows 7+ partially re-introduced that functionality
(just in differencing "Windows=On" from "Off").

See https://reactos.org/wiki/User:Hbelusca/CSRSS for more information.