Undo an accidental change.
Improve a comment.
Update some documentation.
Fix a compiler warning on Linux.
Improvements to libavoid crossing penalty rerouting.

This change chooses a minimal subset of crossing-causing connectors to
reroute (rather than rerouting everythig with crossings).  It also
orders these better to reroute the more constrained connectors first.
This improves the overall crossing rerouting heuristic.