Fix #1373 - suppress touchstart on drag.

This is not ideal because it disables clicking on draggable links, but as we are forced to suppress the touchstart event (before any movement) in order to disable touch-to-drag or pinch-to-zoom, I think this is the only option. Or, we might dispatch a synthetic click event if the touchstart is prevented.

This pull request also fixes the bug where touchend in d3.behavior.zoom calls touchstart, which was causing two quick touchend events to be misinterpreted as a dbltap.