Add a commented out machdep.led_blink entry for DEC 3k systems.

src/etc/etc.alpha/sysctl.conf

@@ -2,3 +2,4 @@
 #machdep.unaligned_fix=0	# 0 - disable fixup of unaligned access
 #machdep.unaligned_sigbus=0	# 0 - don't sigbus on unaligned access
 #machdep.allowaperture=1	# see xf86(4)
+#machdep.led_blink=1		# blink chassis leds on DEC 3000