We cannot blindly use the selected newsgroup for
HEAD/ARTICLE/BODY requests using Message-ID, since
those commands look across all newsgroups; not just
the selected one (if any).
So stuff a reference to the Inbox object into $smsg.
We can reduce args passed into set_nntp_headers() and
msg_hdr_write(), too.
Fixes: 0e6ceff37fc38f28 ("nntp: support slow blob retrievals")