[packagekit] 'native' search filter

[Richard Hughes]

At the moment, we define the 'native' filter as "only return packages
in the computer native architecture" -- this ensures that users don't
"find" i386 packages on x64 and download a *ton* of library deps just
to install them.

This resulted in a RHEL bug, where users on x64 were searching for
acroread and the binary flash player and getting no results. It was
suggested we change the semantics to be: "only return packages in the
computer native architecture when a native architecture package
exists"

Does this make sense? I wanted to sanity check it with you guys.

Richard.