Enable extra debugging.

Route BX_INFO to debug port.
Add a new debug_stub() function.
diff --git a/src/output.c b/src/output.c
index 6c7f719..043caa2 100644
--- a/src/output.c
+++ b/src/output.c
@@ -140,6 +140,7 @@
             puts_cs(action, sarg);
             break;
         default:
+            putc(action, '%');
             putc(action, *s);
             n = s;
         }
@@ -178,3 +179,9 @@
         return;
     dump_regs(fname, "exit", regs);
 }
+
+void
+__debug_stub(const char *fname, struct bregs *regs)
+{
+    dump_regs(fname, "stub", regs);
+}