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/vars.c b/src/vars.c
index edcf1af..de55f14 100644
--- a/src/vars.c
+++ b/src/vars.c
@@ -7,7 +7,7 @@
 
 #include <types/vars.h>
 
-#include <proto/arg.h>
+#include <haproxy/arg.h>
 #include <proto/http_rules.h>
 #include <proto/http_ana.h>
 #include <proto/sample.h>