Move <boost/detail/none_t.hpp> into optional.

It says in the file that it's part of optional.  This file should
actually be deleted - it's a near duplicate of <boost/none_t.hpp> (part
of optional) and doens't appear to be used anywhere, so I assume it was
an accident. But might as well put it in the right place.
Move <boost/detail/workaround.hpp> from detail into config.

As suggested on the mailling list by Stephen Kelly.

It seems like a natural fit to me.
Move <boost/version.hpp> from detail to config.

As suggested by Stephen Kelly on the mailing list.

This file links to the config documentation, and is documented there.
Move <boost/limits.hpp> from detail to config.

As suggested on the list by Stephen Kelly.

I'm not so sure about this one. Compatibility might be a more
appropriate destination as it's documented there, and links to it.
I also included the deleted files <boost/detail/limits.hpp> and
<boost/pending/limits.hpp>.