Introduce the

void backtrace(void);
function which will print a backtrace if DDB is in the kernel and an
explanation if not.

This is useful for recording backtraces in non-fatal circumstances and
does not require pollution with DDB #includes in the files where it
is used.

It would of course be nice to have a non-DDB dependent version too,
but since the meat of a backtrace is MD it is probably not worth it.

sys/kern/kern_shutdown.c

@@ -438,6 +438,20 @@ shutdown_reset(void *junk, int howto)
 	/* NOTREACHED */ /* assuming reset worked */
 }
 
+/*
+ * Print a backtrace if we can.
+ */
+
+void
+backtrace(void)
+{
+#ifdef DDB
+	db_print_backtrace();
+#else
+	printf("Sorry, need DDB option to print backtrace");
+#endif
+}
+
 #ifdef SMP
 static u_int panic_cpu = NOCPU;
 #endif

sys/sys/systm.h

@@ -135,6 +135,7 @@ void	panic(const char *, ...) __printflike(1, 2);
 void	panic(const char *, ...) __dead2 __printflike(1, 2);
 #endif
 
+void	backtrace(void);
 void	cpu_boot(int);
 void	cpu_rootconf(void);
 extern uint32_t crc32_tab[];