window-port updates

Remove start_screen() and end_screen() from the
Window-port interface.

They were only ever used by tty, and there was a comment
carried to several window-ports about how they "really
should go away. They are tty-specific"

term_start_screen() and term_end_screen() are part of
terminal/NO_TERMS supporting routines now.

sys/windows/windsys.c

@@ -231,8 +231,10 @@ VA_DECL(const char *, s)
     VA_START(s);
     VA_INIT(s, const char *);
     /* error() may get called before tty is initialized */
+#ifdef TTY_GRAPHICS
     if (iflags.window_inited)
-        end_screen();
+        term_end_screen();
+#endif
     if (WINDOWPORT(tty)) {
         buf[0] = '\n';
         (void) vsnprintf(&buf[1], sizeof buf - (1 + sizeof "\n"), s, VA_ARGS);