gui-win32: fix encoding and decoding of clipboard strings

On Windows, the clipboard strings are encoded in UTF-16.
However, Drawterm considers them as Rune. It used to work
when a Rune was a short, but a Rune is now a int.
The solution is to implement Rune16 functions to
handle UTF-16 strings.

The Rune16 functions were written by Charles Forsyth
as part of Inferno. Balaji Srinivasa integrated them
into Drawterm.

gui-win32/Makefile

@@ -7,7 +7,8 @@ OFILES=\
 	cload.$O\
 	draw.$O\
 	load.$O\
-	screen.$O
+	screen.$O\
+	r16.$O
 
 default: $(LIB)
 $(LIB): $(OFILES)