Add idle truck count to HUD

The HUD currently shows me the total number of trucks which, while interesting, isn't something I really need to know that much since my truck count is capped anyway. On the other hand, something I am constantly checking for is how many of my unassigned trucks (i.e. those doing general duty) are currently busy/idle. I'd much rather see this information displayed in the HUD rather than having to go into the vehicles panel to retrieve this information, as it will inform me whether I should let trucks various jobs or spend the unity to do it myself.