Fuzz testing debug tool

Add code to run a fuzz tester, simulating (more-or-less) random
keyboard mashing. There's no option to turn it on, you need to
set iflags.debug_fuzzer on via a debugger or something along
those lines.

include/flag.h

@@ -237,6 +237,7 @@ struct instance_flags {
      * behaviour of various NetHack functions and probably warrant
      * a structure of their own elsewhere some day.
      */
+    boolean debug_fuzzer;  /* fuzz testing */
     boolean defer_plname;  /* X11 hack: askname() might not set plname */
     boolean herecmd_menu;  /* use menu when mouseclick on yourself */
     boolean invis_goldsym; /* gold symbol is ' '? */