gutils.c: Improve g_get_os_info() for Windows Server 2008+

Instead of Windows 7/8/8.1 Server, use the proper names Server 2008
R2/2012/2012 R2 so that things are clearer to people.  Since nowadays
GLib requires Windows 7 (_WIN32_WINNT 0x0601, meaning the server
counterpart is Server 2008 R2), we include Server 2008 in the list for
completeness' sake, but exclude the Server 2003/2003R2 from the list.