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.

win/tty/topl.c

@@ -186,6 +186,8 @@ more()
     /* avoid recursion -- only happens from interrupts */
     if (ttyDisplay->inmore++)
         return;
+    if (iflags.debug_fuzzer)
+        return;
 
     if (ttyDisplay->toplin) {
         tty_curs(BASE_WINDOW, cw->curx + 1, cw->cury);