Function to determine whether Server is Core/Nano environment

[Nutzzz]

Requesting a function to determine whether Server is Core/Nano environment.

See https://msdn.microsoft.com/en-us/library/hh846315(v=vs.85).aspx

The method Microsoft recommends for determining this depends on the Windows version. Core was introduced in Server 2008, and they recommend using GetProductInfo, though a registry key is another option (which method I'm currently using).

They recommend checking a different set of registry keys for 2012/2016, though for 2012 at least the registry keys mentioned for 2008 seem to work (and I imagine 2016 is the same, however I'm not sure whether Nano reports itself as Core or not). I'll be checking on that in the next few days. Perhaps that's not important though, because Nano (unfortunately for us) dispenses with the 32-bit emulation layer, so even with the technique recommended by Microsoft, such a determination will not actually be possible unless NSIS is x64 and until/unless this plugin is rebuilt for x64 (and yes, I've read #18 and the difficulties there will be implementing such--though note that the functions I've used in this plugin DO seem to work fine with NSIS 3.0, accidentally or not).