[Exim] vrfy feature request

It would be very useful to have a callout verification option to use
VRFY instead of RCPT when verification is called for. I attempted to put
together a patch to accomplish this, but I'm not quite sure how it
should interact with the random_local_part callout. I'm thinking they
should be mutually exclusive if I'm understanding this correctly. For
the moment, I have hit the limits of my programming ability, so if
anyone can steer me in the right direction I would be grateful. A
working patch would be even better!