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/http_htx.c b/src/http_htx.c
index af4dd5c..8524d8a 100644
--- a/src/http_htx.c
+++ b/src/http_htx.c
@@ -24,7 +24,7 @@
 #include <haproxy/http_htx.h>
 #include <haproxy/htx.h>
 
-#include <proto/arg.h>
+#include <haproxy/arg.h>
 #include <proto/http_fetch.h>
 #include <proto/sample.h>