MEDIUM: dumpstats: Display error message during add of values.

This patch adds new display type. This display returns allocated string,
when the string is flush into buffers, it is freed. This permit to
return the content of "memprintf(err, ...)" messages.

The pat_ref_add functions has changed to return error.
diff --git a/include/proto/pattern.h b/include/proto/pattern.h
index 450a8db..e9269e4 100644
--- a/include/proto/pattern.h
+++ b/include/proto/pattern.h
@@ -174,8 +174,8 @@
 struct pat_ref *pat_ref_newid(int unique_id, const char *display, unsigned int flags);
 int pat_ref_append(struct pat_ref *ref, char *pattern, char *sample, int line);
 int pat_ref_add(struct pat_ref *ref, const char *pattern, const char *sample, char **err);
-int pat_ref_set(struct pat_ref *ref, const char *pattern, const char *sample);
-int pat_ref_set_by_id(struct pat_ref *ref, struct pat_ref_elt *refelt, const char *value);
+int pat_ref_set(struct pat_ref *ref, const char *pattern, const char *sample, char **err);
+int pat_ref_set_by_id(struct pat_ref *ref, struct pat_ref_elt *refelt, const char *value, char **err);
 int pat_ref_delete(struct pat_ref *ref, const char *key);
 int pat_ref_delete_by_id(struct pat_ref *ref, struct pat_ref_elt *refelt);
 void pat_ref_prune(struct pat_ref *ref);