Change the default poweroff delay from 0 to 5 seconds. This seems to be

adequate for the IDE disks that I have available for testing.  Most seem
to wait between 1 and 3 seconds before flushing their caches.

Add the ability to override the delay at compile time via the
undocumented option POWEROFF_DELAY.  The delay can still be set via
sysctl as it was originally implemented.

sys/kern/kern_shutdown.c

@@ -556,7 +556,11 @@ panic(const char *fmt, ...)
 /*
  * Support for poweroff delay.
  */
-static int poweroff_delay = 0;
+#ifndef POWEROFF_DELAY
+# define POWEROFF_DELAY 5000
+#endif
+static int poweroff_delay = POWEROFF_DELAY;
+
 SYSCTL_INT(_kern_shutdown, OID_AUTO, poweroff_delay, CTLFLAG_RW,
 	&poweroff_delay, 0, "");