Implement basic chmod() syscall and /bin/chmod helper.

Only raw octal modes are supported right now.
This patch also changes mode_t from 32-bit to 16-bit to match the on-disk
type used by Ext2FS.

I also ran into EPERM being errno=0 which was confusing, so I inserted an
ESUCCESS in its place.

Kernel/Ext2FileSystem.h

@@ -41,6 +41,7 @@ private:
     virtual int increment_link_count() override;
     virtual int decrement_link_count() override;
     virtual size_t directory_entry_count() const override;
+    virtual bool chmod(mode_t, int& error) override;
 
     void populate_lookup_cache() const;