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/SyntheticFileSystem.h

@@ -65,6 +65,7 @@ private:
     virtual bool remove_child(const String& name, int& error) override;
     virtual RetainPtr<Inode> parent() const override;
     virtual size_t directory_entry_count() const override;
+    virtual bool chmod(mode_t, int& error) override;
 
     SynthFS& fs();
     const SynthFS& fs() const;