Honor the "dumpdev" kenv variable if it is set and the "dumpdev" rc

variable is set to "AUTO".

MFC after:	2 weeks

etc/rc.d/dumpon

@@ -14,6 +14,16 @@ name="dumpon"
 start_cmd="dumpon_start"
 stop_cmd="dumpon_stop"
 
+dumpon_try()
+{
+	if /sbin/dumpon -v "${1}" ; then
+		# Make a symlink in devfs for savecore
+		ln -fs "${1}" /dev/dumpdev
+		return 0
+	fi
+	return 1
+}
+
 dumpon_start()
 {
 	# Enable dumpdev so that savecore can see it. Enable it
@@ -23,20 +33,21 @@ dumpon_start()
 	[Nn][Oo] | '')
 		;;
 	[Aa][Uu][Tt][Oo])
+		dev=$(/bin/kenv dumpdev)
+		if [ -n "${dev}" ] ; then
+			dumpon_try "${dev}"
+			return $?
+		fi
 		while read dev mp type more ; do
 			[ "${type}" = "swap" ] || continue
 			[ -c "${dev}" ] || continue
-			if /sbin/dumpon -v "${dev}" 2>/dev/null; then
-				# Make a symlink in devfs for savecore
-				ln -s ${dev} /dev/dumpdev
-				return 0
-			fi
+			dumpon_try "${dev}" 2>/dev/null && return 0
 		done </etc/fstab
 		echo "No suitable dump device was found." 1>&2
 		return 1
 		;;
 	*)
-		/sbin/dumpon -v ${dumpdev}
+		dumpon_try "${dumpdev}"
 		;;
 	esac
 }