curses menu: support backspace/delete for counts

Have curses call the core get_count() routine instead rolling its
own so that backspace and delete are supported.  That part was
trivial to accomplish.  Unfortunately it brought the disappearing
menu phenomenon back so it became more complicated overall.

win/curses/cursmisc.h

@@ -25,7 +25,7 @@ void curses_prehousekeeping(void);
 void curses_posthousekeeping(void);
 void curses_view_file(const char *filename, boolean must_exist);
 void curses_rtrim(char *str);
-int curses_get_count(int first_digit);
+long curses_get_count(int first_digit);
 int curses_convert_attr(int attr);
 int curses_read_attrs(const char *attrs);
 char *curses_fmt_attrs(char *);