Some rewording of object code distribution condition.

Mainly restores accidental deletion of the 2-year requirement for
network-available source where binaries are distributed in products.
Add 'Copyleft Sunset' provision.

See mailing list thread "Limited-term copyleft" beginning at
https://lists.fedorahosted.org/pipermail/copyleft-next/2013-February/000499.html