lib: Better hashing of string values using Bernstein hash

* hash.{h,c}: (string_hash_make) Hash optimised for strings, current
  implementation using Bernstein hash, which offers a good compromise
  between distribution and performance.
* distribute.c: (distribute_hash_make) use previous instead of additive
  string hash.
* if_rmap.c: (if_rmap_hash_make) ditto
diff --git a/lib/hash.h b/lib/hash.h
index f4b1c23..4cb772e 100644
--- a/lib/hash.h
+++ b/lib/hash.h
@@ -70,4 +70,6 @@
 extern void hash_clean (struct hash *, void (*) (void *));
 extern void hash_free (struct hash *);
 
+extern unsigned int string_hash_make (const char *);
+
 #endif /* _ZEBRA_HASH_H */