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.

sys/windows/win32api.h

@@ -0,0 +1,49 @@
+/* NetHack 3.7	win32api.h	$NHDT-Date: 1596498320 2020/08/03 23:45:20 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.15 $ */
+/* Copyright (c) NetHack PC Development Team 1996                 */
+/* NetHack may be freely redistributed.  See license for details. */
+
+/*
+ * This header file is used to clear up some discrepencies with Visual C
+ * header files & NetHack before including windows.h, so all NetHack
+ * files should include "win32api.h" rather than <windows.h>.
+ */
+#ifndef WIN32API
+#define WIN32API
+
+#if defined(HACK_H)
+#error win32api.h must be included before hack.h
+#endif
+
+#if defined(strcmpi)
+#error win32api.h should be included first
+#endif
+
+#if defined(min)
+#error win32api.h should be included first
+#endif
+
+#if defined(max)
+#error win32api.h should be included first
+#endif
+
+#if defined(Protection)
+#error win32api.h should be included first
+#endif
+
+#if defined(_MSC_VER)
+#pragma warning(disable : 4142) /* Warning, Benign redefinition of type */
+#pragma pack(8)
+#endif
+
+#define WIN32_LEAN_AND_MEAN
+
+#include <windows.h>
+#include <commctrl.h>
+
+#if defined(_MSC_VER)
+#pragma pack()
+#endif
+
+#endif // WIN32API
+
+/*win32api.h*/