I have updated the masking keymap to be functional again. At least in my tests it works good with both configurations, 2.80 and 2.79, both RMB and LMB select.
In the future (hopefully 2.81) the masking tools will be updated anyway, to make use of the 2.8 tool system. Until then we should have an intuitive set of controls for mask creation and manipulation that is close to what we have in 2.79. So this is more a temporary solution anyway.
I have forced LEFTMOUSE for some settings that were bound to params.select_mouse before.
That way we get the same behavior as before, regardless of the mouse button configuration, so rightclickers and leftclickers can use the masking tools in the same way, similar to 2.79, where we also used the left mouse button to tweak the spline and spline curvature, as well as create new mask points.