nntp: make ->ALL Xref generation more fuzzy
For ->ALL users, this mitigates the regression introduced
by commit
811b8d3cbaa790f59b7b107140b86248da16499b
("nntp: xref: use ->ALL extindex if available"), since
it's common to cross post messages to some mailing
lists with per-list trailers for unsubscribe information.
We won't bother dealing with Bcc-ed messages since those
are nearly all spam when it comes to public mailing lists.
Fixes: 811b8d3cbaa790f5 ("nntp: xref: use ->ALL extindex if available")
Link: https://public-inbox.org/meta/20201130194201.GA6687@dcvr/