CODE: factor out calculation of convoy theoretical max speed

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@7396 8aca7d54-2c30-db11-9de9-000461428c89
FIX: check more places for valid besch's

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@7397 8aca7d54-2c30-db11-9de9-000461428c89
sqapi: export more settings

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@7398 8aca7d54-2c30-db11-9de9-000461428c89