MINOR: debug: switch to SIGURG for thread dumps

The current choice of SIGPWR has the adverse effect of stopping gdb each
time it is triggered using "show threads" or example, which is not really
convenient. Let's switch to SIGURG instead, which we don't use either.

src/debug.c

@@ -388,8 +388,11 @@ void ha_thread_dump_all_to_trash()
 
 #else /* below USE_THREAD_DUMP is set */
 
-/* The signal to trigger a debug dump on a thread is SIGPWR */
-#define DEBUGSIG SIGPWR
+/* The signal to trigger a debug dump on a thread is SIGURG. It has the benefit
+ * of not stopping gdb by default, so that issuing "show threads" in a process
+ * being debugged has no adverse effect.
+ */
+#define DEBUGSIG SIGURG
 
 /* mask of threads still having to dump, used to respect ordering */
 static volatile unsigned long threads_to_dump;