Create setNewFileMode function.

Create separate files with setNewFileMode to avoid
runtime checks.

backend/local/local_unix.go

@@ -0,0 +1,12 @@
+// +build !windows
+
+package local
+
+import (
+	"os"
+)
+
+// set file to readonly
+func setNewFileMode(f string, fi os.FileInfo) error {
+	return os.Chmod(f, fi.Mode()&os.FileMode(^uint32(0222)))
+}