[exim] DomainKeys support: CVS snapshot

I have updated the snapshot supporting DomainKeys (DK). It's available here:

http://duncanthrax.net/exim-experimental/

The docs are included in doc/experimental-spec.txt now. Please note the
implementation has changed. If you used the previous snapshot you'll
need to update your configuration. The current implementation is
supposed to be fairly "final", unless the underlying library changes
radically.

/tom