New example in progres : Koda Ruskey (functional correctness done; missing proof of correctness for the number of forest colorings)

Conflicts:
	examples/avl/priority_queue/why3session.xml
	examples/avl/priority_queue/why3shapes.gz

This reverts commit 98f2682f.

For the moment, new API functions are introduced. If existing API
functions are called, the why3server is not used.

* why3config.ml
(plugins_auto) if plug-ins dir is absent, do not attempt to add plug-ins

Function Sysutil.relativize_filename does not work well when its first
argument contains special subpaths for the parent and current directories
(".." and "."). Provide a function Sysutil.normalize_filename that removes
such subpaths, to be called before Sysutil.relativize_filename.