fix cols default on macOS

[cebtenzzre]

On macOS, you can't just pass the string 1234 to this ioctl, it needs to be 8 bytes:

>>> import fcntl, termios
>>> fcntl.ioctl(0, termios.TIOCGWINSZ, "1234")
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    fcntl.ioctl(0, termios.TIOCGWINSZ, "1234")
    ~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
SystemError: buffer overflow

But shutil provides a cross-platform way to get the terminal width, which we should just use.