backup.cgi: Read and deliver the requested backup in pieces

Read-in the requested backup file line by line and directly deliver this
as stream to the client.

This fixes the problem with very big backups on systems with very
limited RAM, which resulted in an OOM kill of the CGI and delivery
process.

Fixes #13096.

Signed-off-by: Stefan Schantl <stefan.schantl@ipfire.org>
Reviewed-by: Michael Tremer <michael.tremer@ipfire.org>

html/cgi-bin/backup.cgi

@@ -366,6 +366,13 @@ sub deliver_file() {
 
 	# Deliver content
 	open(FILE, "<$file") or die "Unable to open $file: $!";
-	print <FILE>;
+
+	# Read the file line by line and send it
+	# to the client.
+	while (<FILE>) {
+		print $_;
+	}
+
+	# Close file handle.
 	close(FILE);
 }