Add is_windows_server_2016

[Richard Cotton]

Slightly tricky because utils::win.version() just calls it "Windows Server", so need to be careful not to match other versions of Windows server. Need a Windows machine to check this properly!

https://github.com/wch/r-source/blob/af7f52f70101960861e5d995d3a4bec010bc89e6/src/library/utils/src/windows/util.c

https://msdn.microsoft.com/en-us/library/windows/desktop/ms724832(v=vs.85).aspx