[Exim] Just a warning to those who use host lists for relay …

A couple days ago, I removed a network from the host_accept_relay file.
That caused a negated entry to become the last. As a result, suddenly
we became an open relay. :(

I know, it's documented in the domain list chapter. It's still easy to
get bitten by this feature. Now I set 127.0.0.1 as the last entry and
put a telling comment there.

I know this is common ACL semantics, but perhaps still something could
be done about it?

Michael