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/vs/build.bat

@@ -0,0 +1,44 @@
+@echo off
+
+if "%VSCMD_VER%"=="" (
+	echo MSBuild environment not set ... attempting to setup build environment.
+	call :setup_environment
+)
+
+if "%VSCMD_VER%"=="" (
+	echo Unable to setup build environment.  Exiting.
+	goto :EOF
+)
+
+msbuild NetHack.sln /t:Clean;Build /p:Configuration=Debug;Platform=Win32
+msbuild NetHack.sln /t:Clean;Build /p:Configuration=Debug;Platform=x64
+msbuild NetHack.sln /t:Clean;Build /p:Configuration=Release;Platform=Win32
+msbuild NetHack.sln /t:Clean;Build /p:Configuration=Release;Platform=x64
+
+goto :EOF
+
+:setup_environment
+
+
+if "%VS150COMNTOOLS%"=="" (
+	call :set_vs15comntools
+)
+
+if "%VS150COMNTOOLS%"=="" (
+	echo Can not find Visual Studio 2017 Common Tools path.
+	echo Set VS150COMNTOOLS appropriately.
+	goto :EOF
+)
+
+call "%VS150COMNTOOLS%VsMSBuildCmd.bat"
+cd %~dp0
+
+goto :EOF
+
+:set_vs15comntools
+
+if exist "%ProgramFiles(x86)%\Microsoft Visual Studio\2017\Professional\Common7\Tools" (
+	set "VS150COMNTOOLS=%ProgramFiles(x86)%\Microsoft Visual Studio\2017\Professional\Common7\Tools\"
+)
+
+goto :EOF