[clear] CSA patch for Exim

I've revised my CSA patch again, to fix a typo in a comment (spotted by
Matthew Elvey), and a bug that was triggered by the lack of a remote IP
address in local SMTP mode.

Abbreviated patch (just the code) at:
http://www.cus.cam.ac.uk/~fanf2/hermes/doc/antiforgery/exim-csa.2b.patch

Tony.
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