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/vms/Install.vms

@@ -42,7 +42,7 @@
         [.sys   .wince]  -- Windows CE
         [.sys   .wince   .ceinc] -- more WinCE
         [.sys   .wince   .ceinc   .sys] -- ditto
-        [.sys   .winnt]  -- Windows NT
+        [.sys   .windows]  -- MS Windows
         [.win   .curses] - window routines for curses interface
         [.win   .gem]    -- window routines for Atari/GEM
         [.win   .gnome]  -- window routines for Unix/GNOME

sys/vms/Makefile.src

@@ -386,7 +386,7 @@ spotless :	clean
 # config.h timestamp
 $(CONFIG_H) : $(INC)config.h $(INC)config1.h $(INC)tradstdc.h $(INC)global.h \
 		$(INC)coord.h $(INC)vmsconf.h $(INC)system.h $(INC)unixconf.h \
-		$(INC)micro.h $(INC)pcconf.h $(INC)ntconf.h
+		$(INC)micro.h $(INC)pcconf.h $(INC)windconf.h
 	$(TOUCH) $(CONFIG_H)
 # hack.h timestamp
 $(HACK_H) : $(INC)hack.h $(CONFIG_H) $(INC)align.h \