Cleans up train latching/unlatching

Also adds a verb for turning off train engines.
Merge remote-tracking branch 'upstream/dev' into train-cleanup

Resolved merge conflicts, and added some more user feedback to attach/unattach procs.

Conflicts:
	code/modules/vehicles/cargo_train.dm
Merge pull request #5320 from mwerezak/train-cleanup

Cleans up train latching/unlatching