Refactor the interface to find_routes().

This function is local to pathfind.cpp, so this has very limited
impact. It will allow a reduction in the data stored in the undo
stack, though.


git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56373 75b3775d-3801-0410-9e12-e2e43121521d