Let root manipulate a fuse-mounted redoxfs vfs.

This (along with a few Makefile changes) solves my issues building on macOS.

fuse/main.rs

@@ -7,6 +7,7 @@ extern crate time;
 
 use image::Image;
 use std::env;
+use std::ffi::OsStr;
 use std::path::Path;
 use time::Timespec;
 use fuse::{FileType, FileAttr, Filesystem, Request, ReplyData, ReplyEntry, ReplyAttr, ReplyCreate, ReplyDirectory, ReplyEmpty, ReplyStatfs, ReplyWrite};
@@ -289,7 +290,14 @@ fn main() {
                     if let Some(mountpoint) = env::args_os().nth(2) {
                         fuse::mount(RedoxFS {
                             fs: filesystem
-                        }, &mountpoint, &[]);
+                        }, &mountpoint, &[
+                            // One of the uses of this redoxfs fuse wrapper is to populate a filesystem
+                            // while building the Redox OS kernel. This means that we need to write on
+                            // a filesystem that belongs to `root`, which in turn means that we need to
+                            // be `root`, thus that we need to allow `root` to have access.
+                            OsStr::new("-o"),
+                            OsStr::new("allow_root"),
+                        ]);
                     } else {
                         println!("redoxfs: no mount point provided");
                     }