Feature: Industries with neutral stations (e.g. Oil Rig) only supply/accept cargo to/from their neutral station. (#7234)

This change is a controlled by a game setting, located under Environment ->
Industries which allows toggling the behaviour. It defaults to enabled.

"Company stations can serve industries with attached neutral stations"

When enabled, industries with attached neutral station (such as Oil Rigs) may
also be served by company-owned stations built nearby. This is the traditional
behaviour.

When disabled, these industries may only be served by their neutral station.
Any nearby company-owned stations won't be able to serve them, nor will the
neutral station serve anything else other than the industry.

src/industry.h

@@ -18,6 +18,7 @@
 #include "industry_map.h"
 #include "industrytype.h"
 #include "tilearea_type.h"
+#include "station_base.h"
 
 
 typedef Pool<Industry, IndustryID, 64, 64000> IndustryPool;
@@ -41,6 +42,7 @@ enum ProductionLevels {
 struct Industry : IndustryPool::PoolItem<&_industry_pool> {
 	TileArea location;                                     ///< Location of the industry
 	Town *town;                                            ///< Nearest town
+	Station *neutral_station;                              ///< Associated neutral station
 	CargoID produced_cargo[INDUSTRY_NUM_OUTPUTS];          ///< 16 production cargo slots
 	uint16 produced_cargo_waiting[INDUSTRY_NUM_OUTPUTS];   ///< amount of cargo produced per cargo
 	uint16 incoming_cargo_waiting[INDUSTRY_NUM_INPUTS];    ///< incoming cargo waiting to be processed