cli/reply: return internet address list from get header funcs

Pass in GMimeMessage to simplify To/Cc/Bcc headers. We'll eventually
remove the notmuch message passing altogether, but keep both for now
to not make too big changes at once.

Getting the headers from GMimeMessage using GMime functions fixes the
error on duplicate Cc headers reported by Daniel Kahn Gillmor
<dkg@fifthhorseman.net> in id:87d1ngv95p.fsf@alice.fifthhorseman.net.

Get rid of an intermediate function.

The small annoyance is the ownership differences in the address lists.