whitespace.el: auto-cleanup on kill prevents killing read-only buffers

[Reuben Thomas]

I have whitespace-global-mode on and have it set to auto-cleanup on kill. 
This stops read-only buffers from being killed, as auto-cleanup tries to 
clean them up, gets an error, and aborts the kill.

The old whitespace mode used to check to see if it was being called in a 
read-only buffer, and didn't try to clean up under those circumstances.

I'm using Emacs 23.0.60.1, but as far as I can tell the problem still exists 
in CVS head.

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