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.

win/win32/mhmap.c

@@ -93,7 +93,7 @@ static void clearAll(PNHMapWindow data);
 #if (VERSION_MAJOR < 4) && (VERSION_MINOR < 4) && (PATCHLEVEL < 2)
 static void nhglyph2charcolor(short glyph, uchar *ch, int *color);
 #endif
-extern boolean win32_cursorblink;       /* from sys\winnt\winnt.c */
+extern boolean win32_cursorblink;       /* from sys\windows\windsys.c */
 
 HWND
 mswin_init_map_window(void)