Bugfix: see dot stderr, drop extra ,box

The generated dot input used `shape=box,box` where the `,box` is
extraneous and rejected as a syntax error, at least in graphviz 2.32.0
(20130816.0025).  Fix that.

Plumb stderr from the invoked dot command through to the program's own
stderr, to see diagnostics.

@davecheney