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/.gitignore

@@ -0,0 +1,17 @@
+!NetHack.sln
+*.db-shm
+*.db-wal
+paniclog
+_pkginfo.txt
+AppPackages
+BundleArtifacts
+report.xml
+*.cer
+*.appxbundle
+NetHackPackage_StoreKey.pfx
+NetHackPackage_TemporaryKey.pfx
+NetHackPackage.assets.cache
+NetHackPackage.wapproj.user
+NetHackPackage/obj/
+NetHackW.vcxproj.user
+obj/