Fix #8981: Don't attempt to re-reserve path if already entering/entered depot. #9000
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Motivation / Problem
When placing a signal on top of a train, an attempt is made to unreserve and re-reserve the path of the train. This is not possible if the head of the train is already inside a depot, as there is no further path to reserve. This flags the train as stuck, requiring manual intervention.
Description
This patch solves this by not attempting to re-reserve the train's path if it is already entering, or has entered a depot.
Limitations
Path signals. Here lie dragons.
Checklist for review
Some things are not automated, and forgotten often. This list is a reminder for the reviewers.