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/xy.h

@@ -34,8 +34,7 @@
 #ifndef XY_H
 #define XY_H
 
-#include "sect.h"
-#include "nat.h"
+#include "types.h"
 
 /* Used to calculate an offset into an array.  Used for
    dynamically sizing the world. */