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.

src/stream_interface.c

@@ -20,6 +20,7 @@
 #include <sys/types.h>
 
 #include <haproxy/api.h>
+#include <haproxy/connection.h>
 #include <haproxy/dynbuf.h>
 #include <haproxy/http_htx.h>
 #include <haproxy/tools.h>
@@ -29,7 +30,6 @@
 
 #include <proto/applet.h>
 #include <proto/channel.h>
-#include <proto/connection.h>
 #include <haproxy/pipe.h>
 #include <proto/proxy.h>
 #include <proto/stream.h>