Police Volkswagen T2 can only be built in a convoy

**Steps to reproduce**

1. Start an Extended game in 1975.
2. Build a road depot.
3. Open Trucks tab.
4. `Police Volkswagen T2` does not appear due to [an unrelated bug in Extended](https://forum.simutrans.com/index.php/topic,21637.0.html), so click `Show all`.

**Expected results**

`Police Volkswagen T2` appears; it can be built and started.

**Actual results**

`Police Volkswagen T2` still does not appear.
Change to `Insert` mode and it appears. It can be built only in a convoy with a Volkswagen Beetle.


**Comments**

Maybe this was intended as an imitation of police prison convoys, but if so it was not intuitive to me as a newbie to the pakset.

This was observed in the 0.6 RC release; I have not tested the latest GitHub commit.