Fix the background half of readahead

We used to rely on shell operations to perform the second half part of readhead
in the background. This is not reliable anymore, probably due to changes in
the shell being used or live-boot.

So we now use start-stop-daemon instead.

config/chroot_local-includes/lib/live/config/0000-readahead

@@ -24,22 +24,24 @@ Readahead ()
 
 Start_readahead ()
 {
-	FG_FILES="$(sed -n "\:$BACKGROUND_AT:q;p" "$READAHEAD_LIST")"
-	BG_FILES="$(sed -n "\:$BACKGROUND_AT:,\$p" "$READAHEAD_LIST")"
+	FG_FILES="sed -n '\:$BACKGROUND_AT:q;p' '$READAHEAD_LIST'"
+	BG_FILES="sed -n '\:$BACKGROUND_AT:,\$p' '$READAHEAD_LIST'"
 	FG_SIZE=$(
 	 cd /
-	 echo "$FG_FILES" |
+	 "$FG_FILES" |
 	 xargs du -c 2>/dev/null |
 	 awk '$2 ~ /^total$/ { t = t + $1 } END { print t }')
 	(cd /
-	 echo "$BG_FILES" |
+	 "$BG_FILES" |
 	 xargs stat >/dev/null 2>/dev/null || :)
 	(cd /
-	 echo "$FG_FILES" |
+	 "$FG_FILES" |
 	 xargs cat 2>/dev/null |
 	 pv -f -s ${FG_SIZE}k >/dev/null || :)
 	(cd /
-	 echo "$BG_FILES" | xargs cat >/dev/null 2>&1 || :) &
+	 start-stop-daemon \
+		--start --background --pid /var/run/background-readahead.pid --startas /bin/sh -- \
+		-c "$BG_FILES | xargs cat >/dev/null 2>&1")
 
 	# Creating state file
 	touch /var/lib/live/config/readahead