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

Almost no changes, removed stdlib and added buf-t and connection-t to
the types to avoid a warning.
diff --git a/src/applet.c b/src/applet.c
index 760ddc3..36607aa 100644
--- a/src/applet.c
+++ b/src/applet.c
@@ -16,10 +16,10 @@
 #include <haproxy/applet.h>
 #include <haproxy/api.h>
 #include <haproxy/list.h>
+#include <haproxy/stream_interface.h>
 #include <haproxy/task.h>
 #include <proto/channel.h>
 #include <proto/stream.h>
-#include <proto/stream_interface.h>
 
 unsigned int nb_applets = 0;