MINOR: debug: implement ha_panic()

This function dumps all existing threads using the thread dump mechanism
then aborts. This will be used by the lockup detection and by debugging
tools.
diff --git a/include/common/debug.h b/include/common/debug.h
index 4f3baed..1038041 100644
--- a/include/common/debug.h
+++ b/include/common/debug.h
@@ -87,6 +87,7 @@
 void ha_task_dump(struct buffer *buf, const struct task *task, const char *pfx);
 void ha_thread_dump(struct buffer *buf, int thr, int calling_tid);
 void ha_thread_dump_all_to_trash();
+void ha_panic();
 
 /* This one is useful to automatically apply poisonning on an area returned
  * by malloc(). Only "p_" is required to make it work, and to define a poison