CORE-6812 ; see also commit ff0951356 (r66688)
This function "just" changes the per-user SCRNSAVE.EXE registry value
to point to the new specified file, changes the SPI values and opens
the desk.cpl "Screensaver" property tab for letting the user change
the new selected screensaver properties.
In particular, it does *NOT* copy the specified file to System32 or
anything else (and doesn't verify that it is a valid PE executable).
Our previous implementation did none of that, and was also relying on
some private setupapi functions.
We now behave closer to Windows' desk.cpl.
Additionally:
- ReactOS-specific feature (compile-time define) disabled by default:
Verify that the specified file actually exists, before changing
the screensaver.
- Use NDEBUG, disabling DPRINT by default;
- Improve InstallScreenSaver[A,W] spec entries;
- Remove NUM_APPLETS define and use _countof() instead.
