Policy Plus - Check for functional Group Policy infrastructure

Last time I worked with Policy Plus, I found that there isn't a clean way of detecting whether the current Windows installation has functional Group Policy infrastructure. That led to Policy Plus failing to actually apply the POL settings to the Registry in certain environments. I was hesitant to use a table of SKUs because new Windows editions might add new entries, so my table would need to be continually maintained. Today, though, I noticed on that MSDN page that GetProductInfo will map the current operating system product to a product that existed when the supplied OS version did. I took advantage of that get a product that Vista knew about, thereby covering all Windows versions that Policy Plus can run on.

I also found another bug: PolFile.WillDeleteValue incorrectly interpreted a deleter value's presence to mean that the value will not be deleted, which is obviously a problem. I corrected that and published these fixes to GitHub.