change references from winnt to windows

rename sys/winnt to sys/windows
move vs (visual studio) folder out of win/win32 and into sys/windows
rename include/ntconf.h to include/windconf.h
rename winnt.c to windsys.c
place visual studio projects into individual subfolders.

This will hopefully resolve GitHub issue #484 as well.

util/mdgrep.h

@@ -230,11 +230,6 @@ static struct grep_var grep_vars[] = { { "0", 0 },
 #else
                                        { "WIN32_PLATFORM_WFSP", 0 },
 #endif
-#if defined(WINNT)
-                                       { "WINNT", 1 },
-#else
-                                       { "WINNT", 0 },
-#endif
 #if defined(WIN_CE)
                                        { "WIN_CE", 1 },
 #else