Train fixes and rebalance

- Fixes #5850
- Fixes #5840
- Fixes #5821

Also fixed a few other bugs and did a little rebalancing:
- Slowed trains to 90% of walking speed (5% nerf)
- Capped emagged train impact damage at 10 (run over damage remains unchanged)
- Mobs are now stunned when they are thrown off a destroyed train
- More fix details in the commit logs

DOWNSTREAM:
I removed the "standing_mob" variable as standing is now handled by whether the mob can stand or not (note: the changes to mob.dm).
I also added a mob_offset_y var so mobs have a different pixel_y offset to other items.