Nest: Implement BGP path mask loop operator

Implement regex-like '+' operator in BGP path masks to match previous
path mask item multiple times. This is useful as ASNs may appear
multiple times in paths due to path prepending for traffic engineering
purposes.

nest/attrs.h

@@ -61,6 +61,7 @@ static inline struct adata *as_path_prepend(struct linpool *pool, const struct a
 #define PM_ASN_EXPR	3
 #define PM_ASN_RANGE	4
 #define PM_ASN_SET	5
+#define PM_LOOP		6
 
 struct f_path_mask_item {
   union {