Split patent section into two sections.

Split patent section into two sections.

GNU GPLv3 section 11 (minus the MS coupon paragraph which was
previously deleted in GPL.next) is concerned with two distinct topics,
patent licenses granted explicitly or by implication from upstream
participants, and third-party patent licenses/covenants granted to
distributors or their customers. GNU GPLv3 sec. 11 has occasionally
been criticized by lawyers for starting out with the grant of a patent
license and then proceeding to define "patent license" for purposes of
subsequent paragraphs.

The main motivation of splitting the section up is to make it easier
to reason about and make changes to each part.