MINOR: global: export many symbols.

The functions "val_payload_lv" and "val_hdr" are useful with
lua. The lua automatic binding for sample fetchs needs to
compare check functions.

The "arg_type_names" permit to display error messages.
diff --git a/src/arg.c b/src/arg.c
index e609ac5..e212f6b 100644
--- a/src/arg.c
+++ b/src/arg.c
@@ -17,7 +17,7 @@
 #include <common/standard.h>
 #include <proto/arg.h>
 
-static const char *arg_type_names[ARGT_NBTYPES] = {
+const char *arg_type_names[ARGT_NBTYPES] = {
 	[ARGT_STOP] = "end of arguments",
 	[ARGT_UINT] = "unsigned integer",
 	[ARGT_SINT] = "signed integer",