emerge: add --changed-deps/--binpkg-changed-deps

The @changed-deps set is useful, but it has limitations similar to the
@installed set (see bug #387059), which can make it unsuitable for use
when updating the whole system. Therefore, implement two new options
that are analogous to --newuse and --binpkg-respect-use, called
--changed-deps and --binpkg-changed-deps.

The rationale for having a separate --binpkg-* option is the same in
both cases: depending on the situation, people may want different
behavior for binary packages. For example, just like
---binpkg-respect-use is automatically enabled if the user has not
specified --usepkgonly, so is --binpkg-changed-deps (though the user
can explicitly override the automatic behavior). In both cases,
inconsistencies in dependencies are automatically avoided, increasing
the probability of a successful dependency calculation.