- use the double-buffer buffer to allow redrawing on resize events.
specifing -d on the command line now only disables synchronous
drawing events.  

- use threaded mouse and keyboard to allow for asynchronous
receoption of quit messages.  this allows plot to exit before drawing
is completed.  for programs like mapdemo, this is important.

there were two things that needed to get fixed as a result
- replace fprint(2, ...); exits("bad"); with sysfatal.  also get rid
of stdio.

- dpoint needed a mach-dependentent (sic) version.  otherwise
points on a resized screen will not be properly placed.