[exim] libspf testers.

I don't use libspf2 for many reasons, but Exim doesn't support Couzen's
libspf, so I have provided a howto here:

http://www.ricky-chan.co.uk/exim/ (same link from www.libspf.org site).

I have also included a patch for libspf so it handles recursions
correctly when the recursive SPF don't pass. (see Recently Changes)

However I have been lax in testing since Exim 4.51, so any person using
libspf (as opposed to libspf2) with Exim, feedback will be appreciated.

Thanks.


Ricky