racket/gui cocoa: toolbar-button is for 10.6 and earlier, only

In particular, 'toobar-button (before 10.7) and 'fullscreen-button
(10.7 and later) do not get along, so use only one of them.
further dehackify .toptoclink style in Scribble CSS
pkg/path: fix for when the argument = a pkgs install directory