[Exim] New FreeBSD port maintainer

Hi folks,

I've handed the role of maintainer of the FreeBSD port for Exim over to
Oliver Eikemeier.

Oliver has been doing great work on not only the Exim port, but also
other ports with which it interacts (e.g. clamd).

This change is a good thing for Exim on FreeBSD. Updates should happen
faster, since I'll no longer be delaying Oliver's excellent
contributions. :-)

Ciao,
Sheldon.