[exim] Time for another release

We're over forty entries in the change-log file since the 4.89
release, so I think it's time for another.

Barring objections or other volunteers I'll kick the process
off soon. As usual there will be a sequence of Release Candidates,
the production of which will be notified for anyone wishing to
test. Feature-additions will still be accepted for the initial
one or two RCs, after which we'll go to bugfixes only for the
remainder of RCs before the final product.
- --
Cheers,
Jeremy