force layout: properly UNSET the .fixed bits: user may be using further ...

...bits and those should be kept intact.

(I use the force layout in several places where it was/is extremely handy to use the higher bits for my own augmentations, which depend on the internal 'if (node.fixed) { dont_move_node }' force.layout logic. 

One such example is http://bl.ocks.org/3616279 which is a layout which adds 'pinning' baheviour where nodes stick to the dragged-to location when you keep SHIFT or CTRL pressed when stopping the drag (when you try that gist, it has the known issue that it doesn't properly keep/release the pinned state; that's an irrelevant detail of that specific example.)