REORG: include: move connection.h to haproxy/connection{,-t}.h

The type file is becoming a mess, half of it is for the proxy protocol,
another good part describes conn_streams and mux ops, it would deserve
being split again. At least it was reordered so that elements are easier
to find, with the PP-stuff left at the end. The MAX_SEND_FD macro was moved
to compat.h as it's said to be the value for Linux.
diff --git a/include/proto/stream_interface.h b/include/proto/stream_interface.h
index 98ccb28..e381d84 100644
--- a/include/proto/stream_interface.h
+++ b/include/proto/stream_interface.h
@@ -25,11 +25,11 @@
 #include <stdlib.h>
 
 #include <haproxy/api.h>
+#include <haproxy/connection.h>
 #include <types/stream.h>
 #include <types/stream_interface.h>
 #include <proto/applet.h>
 #include <proto/channel.h>
-#include <proto/connection.h>
 
 
 extern struct si_ops si_embedded_ops;