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/stub-pdcscrn.c

@@ -0,0 +1,15 @@
+/*
+ * Use this stub to insert a needed pragma prior
+ * to compiling $(PDCWINCON)\pdcscrn.c in the
+ * PDCurses distribution.
+ *
+ * We don't want to ask NetHack builders to
+ * modify the PDCurses distribution in any way,
+ * we only ask that they have the original intact
+ * PDCurses source tree available during the build.
+ *
+ */
+
+#pragma warning(disable : 4996)
+#include "pdcscrn.c"
+