CHG: use finer granularity to check for no activity as we have history for 24 months now

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6436 8aca7d54-2c30-db11-9de9-000461428c89
CHG: increase limits for remove_dummy_player_months / unprotect_abondoned_player_months

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6437 8aca7d54-2c30-db11-9de9-000461428c89