lib: consider all instances of Delivered-To header

lib: consider all instances of Delivered-To header

When using notmuch-reply and guessing the From: address from
Delivered-To headers, I had the wrong address chosen today. This was
because the messages from the notmuch list contain these headers in this
order:

Delivered-To: hannu.hartikainen@gmail.com
...
Delivered-To: hannu@hrtk.in
In my .notmuch-config I have the following configuration:

primary_email=hannu@hrtk.in
other_email=hannu.hartikainen@gmail.com;...

Before this change, notmuch-reply would guess From: @gmail.com because
that is the first Delivered-To header present. After the change, the
primary address is chosen as I would expect.