Let /altroot work with a duid-based fstab.

Reported by & fix tested by Dave Anderson. Thanks!
ok deraadt@

src/etc/daily

@@ -1,5 +1,5 @@
 #
-#	$OpenBSD: daily,v 1.72 2011/12/06 21:02:39 halex Exp $
+#	$OpenBSD: daily,v 1.73 2012/02/08 18:22:43 krw Exp $
 #	From: @(#)daily	8.2 (Berkeley) 1/25/94
 #
 # For local additions, create the file /etc/daily.local.
@@ -90,20 +90,20 @@ if [ -f /var/account/acct ]; then
 fi
 
 # If ROOTBACKUP is set to 1 in the environment, and
-# if filesystem named /altroot is type ffs, on /dev/* and mounted "xx",
+# if filesystem named /altroot is type ffs and mounted "xx",
 # use it as a backup root filesystem to be updated daily.
 next_part "Backing up root filesystem:"
 while [ "X$ROOTBACKUP" = X1 ]; do
-	rootbak=`awk '$2 == "/altroot" && $1 ~ /^\/dev\// && $3 == "ffs" && \
-	    $4 ~ /xx/ \
-		{ print substr($1, 6) }' < /etc/fstab`
+	rootbak=`awk '$2 == "/altroot" && $3 == "ffs" && $4 ~ /xx/ \
+		{ print $1 }' < /etc/fstab`
 	if [ -z "$rootbak" ]; then
 		echo "No xx ffs /altroot device found in the fstab(5)."
 		break
 	fi
-	bakdisk=${rootbak%[a-p]}
+	rootbak=${rootbak#/dev/}
+	bakdisk=${rootbak%%?(.)[a-p]}
 	sysctl -n hw.disknames | grep -Fqw $bakdisk || break
-	bakpart=${rootbak#$bakdisk}
+	bakpart=${rootbak##$bakdisk?(.)}
 	baksize=`disklabel $bakdisk 2>/dev/null | \
 		awk -v "part=$bakpart:" '$1 == part { print $2 }'`
 	rootdev=`mount | awk '$3 == "/" && $1 ~ /^\/dev\// && $5 == "ffs" \