added custom sell value trait. this fixes sell/repair on FACT, and buy/sell exploit on PROC

OpenRa.Game/Traits/CustomSellValue.cs

@@ -0,0 +1,17 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+namespace OpenRa.Traits
+{
+	// allow a nonstandard sell/repair value to avoid
+	// buy-sell exploits like c&c's PROC.
+
+	class CustomSellValueInfo : StatelessTraitInfo<CustomSellValue>
+	{
+		public readonly int Value = 0;
+	}
+
+	class CustomSellValue {}
+}