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

Almost no change was needed; chunk.h was replaced with buf-t.h.
It dpeends on types/vars.h and types/protocol_buffers.h.
diff --git a/src/51d.c b/src/51d.c
index d6033f3..ceff828 100644
--- a/src/51d.c
+++ b/src/51d.c
@@ -8,7 +8,7 @@
 #include <haproxy/http_htx.h>
 #include <haproxy/thread.h>
 #include <types/global.h>
-#include <proto/arg.h>
+#include <haproxy/arg.h>
 #include <proto/http_fetch.h>
 #include <proto/log.h>
 #include <proto/http_ana.h>