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/arg.c b/src/arg.c
index 9c4fa44..467929b 100644
--- a/src/arg.c
+++ b/src/arg.c
@@ -17,7 +17,7 @@
 #include <haproxy/tools.h>
 #include <haproxy/chunk.h>
 #include <types/global.h>
-#include <proto/arg.h>
+#include <haproxy/arg.h>
 
 const char *arg_type_names[ARGT_NBTYPES] = {
 	[ARGT_STOP] = "end of arguments",