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/host.c

@@ -40,12 +40,11 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <sys/types.h>
 #ifndef _WIN32
 #include <arpa/inet.h>
-#include <sys/socket.h>
-#include <netinet/in.h>
 #include <netdb.h>
+#include <netinet/in.h>
+#include <sys/socket.h>
 #include <unistd.h>
 #else
 #include <io.h>