Break inclusion cycle: prototypes.h and commands.h included each

other.  Ensure headers in include/ can be included in any order
(except for econfig-spec.h, which is special).  New header types.h to
help avoid inclusion cycles.  Sort include directives.  Remove some
superflous includes.

include/commodity.h

@@ -36,6 +36,9 @@
 #ifndef COMMODITY_H
 #define COMMODITY_H
 
+#include "types.h"
+#include "item.h"
+
 struct comstr {
     short ef_type;
     natid com_owner;