Support the third (now preferred) method of setting DPI awareness.

We will now try SetProcessDpiAwarenessContext() first, then
SetProcessDpiAwareness() second, and finally SetProcessDPIAware() [sic].

Also, "handle" the WM_DPICHANGED event.

This may solve problems initializing SyncTERM on systems with two
displays that have different DPI settings.