Add RedoxFS as submodule. Do not create passwd if no users are provided

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "redoxfs"]
+	path = redoxfs
+	url = https://github.com/redox-os/redoxfs.git

src/install.rs

@@ -189,7 +189,9 @@ pub fn install(config: Config, cookbook: Option<&str>) -> Result<(), String> {
 
         passwd.push_str(&format!("{};{};{};{};{};{};{}\n", username, password, uid, gid, name, home, shell));
     }
-    file!("etc/passwd", passwd.as_bytes());
+    if ! passwd.is_empty() {
+        file!("etc/passwd", passwd.as_bytes());
+    }
 
     Ok(())
 }