Add sync() syscall and a /bin/sync.

It walks all the live Inode objects and flushes pending metadata changes
wherever needed.

This could be optimized by keeping a separate list of dirty Inodes,
but let's not get ahead of ourselves.

VirtualFileSystem/VirtualFileSystem.h

@@ -123,6 +123,8 @@ public:
 
     InodeIdentifier root_inode_id() const;
 
+    void sync();
+
 private:
     friend class FileDescriptor;
     friend class Vnode;