On load, use previous local company or first usable company

Instead of always using the first company slot

src/company_cmd.cpp

@@ -44,6 +44,7 @@ void ClearEnginesHiddenFlagOfCompany(CompanyID cid);
 
 CompanyID _local_company;   ///< Company controlled by the human player at this client. Can also be #COMPANY_SPECTATOR.
 CompanyID _current_company; ///< Company currently doing an action.
+CompanyID _loaded_local_company; ///< Local company in loaded savegame
 Colours _company_colours[MAX_COMPANIES];  ///< NOSAVE: can be determined from company structs.
 CompanyManagerFace _company_manager_face; ///< for company manager face storage in openttd.cfg
 uint _next_competitor_start;              ///< the number of ticks before the next AI is started
@@ -1184,6 +1185,20 @@ int CompanyServiceInterval(const Company *c, VehicleType type)
 	}
 }
 
+/**
+ * Get the default local company after loading a new game
+ */
+CompanyID GetDefaultLocalCompany()
+{
+	if (_loaded_local_company >= COMPANY_FIRST && _loaded_local_company < MAX_COMPANIES && Company::IsValidID(_loaded_local_company)) {
+		return _loaded_local_company;
+	}
+	for (CompanyID i = COMPANY_FIRST; i < MAX_COMPANIES; i++) {
+		if (Company::IsValidID(i)) return i;
+	}
+	return COMPANY_FIRST;
+}
+
 /**
  * Get total sum of all owned road bits.
  * @return Combined total road road bits.