Change title of section 6 to "Pass-Through".

The previous term, "direct licensing", was one I learned from Till
Jaeger and Axel Metzger in the GPLv3 process. GPLv3 itself uses
"automatic licensing" in the title of section 10. I have heard at
least one US lawyer refer to the automatic licensing provision of GPL
as a "pass through" and I have decided to adopt such terminology in
the title.