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/ioqueue.h

@@ -34,6 +34,11 @@
 #ifndef IOQUEUE_H
 #define IOQUEUE_H
 
+#ifndef _WIN32
+#include <sys/uio.h>
+#endif
+#include "queue.h"
+
 struct io {
     struct emp_qelem queue;
     int size;