Fix github issue #192

sb-sys:enable-interrupt does not exist on SBCL +win32
so disable it on Windows.

* swank/sbcl.lisp (add-sigio-handler, remove-sigio-handlers):
Disable it on Windows along with install-sigint-handler.
* swank/cmucl.lisp, swank/sbcl.lisp: Whitespace cleanup.
* swank/cmucl.lisp: Require at least version 20c and remove some

code that was there only for compatibility with older versions.