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.

src/do.c

@@ -1061,6 +1061,8 @@ doup()
         return 1;
     }
     if (ledger_no(&u.uz) == 1) {
+        if (iflags.debug_fuzzer)
+            return 0;
         if (yn("Beware, there will be no return! Still climb?") != 'y')
             return 0;
     }