ip6: turn RA acceptance off when RAs shouldn't be used (rh #588163)

Make sure we don't inadvertenly let the kernel assign an RA address
when connections that don't allow RA are used.

src/NetworkManagerUtils.h

@@ -73,4 +73,8 @@ void        value_hash_add_bool        (GHashTable *hash,
 
 gboolean nm_utils_do_sysctl (const char *path, const char *value);
 
+gboolean nm_utils_get_proc_sys_net_value (const char *path,
+                                          const char *iface,
+                                          guint32 *out_value);
+
 #endif /* NETWORK_MANAGER_UTILS_H */