Bug #266454 - Make @world an all-inclusive set once again, like it was prior

Bug #266454 - Make @world an all-inclusive set once again, like it was prior
to portage-2.2_rc* releases. In addition to @system, @world now includes a
@selected set which represents user-selected "world" packages and sets that
saved in /var/lib/portage/world{,sets}.

svn path=/main/trunk/; revision=14614