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/extern.h

@@ -172,6 +172,7 @@ E boolean NDECL(status_hilite_menu);
 
 /* ### cmd.c ### */
 
+E char NDECL(randomkey);
 E int NDECL(doconduct);
 E int NDECL(domonability);
 E char FDECL(cmd_from_func, (int NDECL((*))));