[Exim] please get rid of sender_verify_fixup

The sender_verify_fixup option is horribly broken and should _NOT_
exist or, at the very least, should have a big warning about it in the
documentation so people aren't tricked into using it without knowing the
nasty consequences and how badly it violates accepted practice.

It is explicitly designed to take addresses from the headers and stick
them in the envelope sender. That is a big no-no, since it causes mail
loops in cases where the envelope sender was explicitly set to avoid them.

This is especially true with qmail, since it uses an invalid (on purpose)
envelope sender of "#@[]" for what it calls "double bounces". This
invalid address is designed to ensure there is no way the message can
bounce back and cause another bounce, etc. and create a loop. You can
argue all you want about it using an invalid envelope sender, but the fact
is that the sender_verify_fixup option is what is broken in a loop-causing
way by taking information from the headers of the message. It is not
appropriate for a mailer to be trying to make such guesses in areas that
have such critical consequences if you guess wrong.

Thanks.