tree-wide: use "polkit" to refer to PolicyKit/polkit

tree-wide: use "polkit" to refer to PolicyKit/polkit

Back in 2012 the project was renamed, see the release notes for v 0.105
[https://cgit.freedesktop.org/polkit/tree/NEWS#n754]. Let's update our
documentation and comments to do the same. Referring to PolicyKit is confusing
to users because at the time the polkit api changed too, and we support the new
version. I updated NEWS too, since all the references to PolicyKit there were
added after the rename.

"PolicyKit" is unchanged in various URLs and method call names.