Uncommenting machdep.apmhalt should have an effect.

ok marc@ fgsch@

src/etc/etc.i386/sysctl.conf

@@ -1,6 +1,6 @@
 #machdep.allowaperture=2	# See xf86(4)
 #machdep.apmwarn=10		# battery % when apm status messages enabled
-#machdep.apmhalt=0		# 1=powerdown hack, try if halt -p doesn't work
+#machdep.apmhalt=1		# 1=powerdown hack, try if halt -p doesn't work
 #machdep.kbdreset=1		# permit console CTRL-ALT-DEL to do a nice halt
 #machdep.userldt=1		# allow userland programs to play with ldt,
 				# required by some ports