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/src/session.c b/src/session.c
index efc1335..2b52ec8 100644
--- a/src/session.c
+++ b/src/session.c
@@ -11,6 +11,7 @@
  */
 
 #include <haproxy/api.h>
+#include <haproxy/connection.h>
 #include <haproxy/global.h>
 #include <haproxy/http.h>
 #include <haproxy/listener.h>
@@ -20,7 +21,6 @@
 
 #include <types/session.h>
 
-#include <proto/connection.h>
 #include <proto/log.h>
 #include <proto/proxy.h>
 #include <proto/session.h>