David Nickerson reports odd behaviour involving a drag start point

David Nickerson reports odd behaviour involving a drag start point
persisting between separate mouse actions. Revamp all uses of the
ndragcoords field in an attempt to stamp that out: we now distinguish
between active drags (>0), a valid click but no drag yet (0), and a
totally invalid situation in which all mouse activity will be ignored
until the next fresh attempt (-1).

[originally from svn r9405]