Changes

Prefab model unit '''prefab_model''' (found in '''"/def/world/prefab.<infix>.sii"''') got new attribute '''allowed_trailer_length''' defining maximum length of trailer or trailer chain that can be spawned or delivered to the company prefab (value has to be integer in range [0, 255] and presents length in meters). When not specified or set to 0 then no restriction is applied. After changing these values map has to be re-saved to propagate values to company instances correctly.