Use AUTHORS instead of AUTHOR if we have multiple people to

Use AUTHORS instead of AUTHOR if we have multiple people to
attribute. Also, fixed checking such that we generate author
section even if we don't have an author (as long as there is at
least one other person/entity we can put in the section). Also
adjusted assembly of content for Author metainfo field such that
we now not only use author, but try to find a "best match" if we
can't find an author name to put there.

Closes #1233592. Thanks to Sam Steingold for the request.