Accessibility: option to prevent status line updates

Apparently some screen readers keep reading the status lines
at the bottom of the screen when parts of those change.
Add an option to prevent updates to those lines.

include/flag.h

@@ -232,6 +232,7 @@ struct instance_flags {
     boolean echo;             /* 1 to echo characters */
     boolean use_menu_color;       /* use color in menus; only if wc_color */
     boolean use_status_hilites;   /* use color in status line */
+    boolean status_updates;   /* allow updates to bottom status lines */
     boolean use_background_glyph; /* use background glyph when appropriate */
     boolean hilite_pile;          /* mark piles of objects with a hilite */
     boolean autodescribe;     /* autodescribe mode in getpos() */