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.

src/client/ioqueue.h

@@ -35,6 +35,8 @@
 #ifndef IOQUEUE_H
 #define IOQUEUE_H
 
+#include "queue.h"
+
 struct ioqueue {
     struct qelem queue;		/* queue fwd/back */
     int bsize;			/* basic block size */