Begin unsquashfs module, port copy operation from Thus.

What case does this `if` protect against? If `mkdir` fails, it should except:

    >>> os.mkdir('test')
    >>> os.mkdir('test')
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    OSError: [Errno 17] File exists: 'test'

Or

    >>> os.mkdir('test/nopermissions')
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    OSError: [Errno 13] Permission denied: 'test/nopermissions'

It might make more sense to wrap the `mkdir` call in a `try/except`.