Add this both as module an input parameter. Complements already-existing
config param notifyonconnectionclose and mirrors the similar feature from
imptcp.
The module parameter acts as default, similarly to notifyonconnectionclose.
Note that in contrast to imptcp, we emit IP addresses and not host
names. This sticks with the traditional semantics of imtcp.
Note that we also fixed a mislading error message in the case when a
disallowed sender tried to connect.
Thanks to John Chivian for suggesting the addition.
