Generalize child-of-fair-use clause further.

As proposed by Mike Linksvayer
https://lists.fedorahosted.org/pipermail/copyleft-next/2013-February/000492.html
Limit section 10 to GPL compatibility.

It is not particularly useful or necessary to provide for outbound
AGPLv3+ compatibility (if we assume that future versions of the GPL
and AGPL will continue to be compatible to at least the same extent
GPLv3 and AGPLv3 are, an assumption that seems reasonable to make).

Since copyleft-next is outbound GPLv3 compatible, it is by definition
outbound AGPLv3 compatible. This allows for simplification of this
provision. Effective AGPLv3 compatibility can be noted in a FAQ.

Any value in specifically noting AGPL compatibility seems to be purely
political in nature. It is already a basic principle of copyleft-next
drafting that mere political benefit is never a good enough reason for
retention or inclusion of a license provision.

But that political value seems rather negligible in any case. AGPLv3
sees little use, most of it by proprietary relicensing companies or
entrepreneurs engaging in a licensing tactic that copyleft-next is
designed to discourage. By contrast, GPLv3 is a very widely used
license.
Incorporate outbound GPL compatibility into basic copyleft provision.

This commit deletes the 'Outbound GPL Compatibility' section and adds
a new paragraph to section 5, influenced by MPL 2.0's section 3.3.

With this change, outbound GPL compatibility is expressed as an
optional supplementary act that can be done when satisfying the
copyleft requirement. Distributed Derived Works must be licensed under
copyleft-next, but 'You' can *additionally* license the Derived Work
under any version of the GPL.

One thing that has changed here is that there is no explicit
permission to 'relicense' the *Received Work* (what You received,
unmodified, at least conceptually) under the GPL. It does not seem to
be necessary (much as the drafters of MPL 2.0 did not seem to regard
it as necessary to achieve downstream GPL/LGPL/AGPL compatibility
outside of the context of what the MPL calls 'Larger Works'). In any
case where outbound GPL compatibility is thought of as desirable or
necessary, we should be within the definition of 'Derived Work'.