mscgen (http://www.mcternan.me.uk/mscgen/) seems much more advanced
than gen_ladder, so I'm creating new diagrams in that language instead.
What remains to be done is a converter to conver the legacy ladder diagrams
to mscgen format.
This version generates PNG graphics based on the perl GD bindings and no
longer uses dot craphics. This solves the long-standing problems that
existed with dot and 'bent arrows'
* re-add support for dashed arrows
* re-add support for bi-directional arrows