Mail headers can contain multiple headers of any type, so ensure
we don't hide any information we're getting in the per-message
permalink views.
This means it's possible to have multiple From, Date, To, Cc,
Subject, and In-Reply-To headers displayed.
The thread indices are a special case, I guess, since we run
out of space on the line if the headers too long and tools like
mutt only show the first one.