netutils: get interface by name rather than ID

The zone ID part in a host part is the name of the network interface
(which by chance is the string representation of the ID with GNRC), not
the ID.

sys/net/netutils/util.c

@@ -56,7 +56,7 @@ int netutils_get_ipv6(ipv6_addr_t *addr, netif_t **netif, const char *hostname)
     size_t len = strlen(hostname);
     char *iface = strchr(hostname, '%');
     if (iface) {
-        *netif = netif_get_by_id(atoi(iface + 1));
+        *netif = netif_get_by_name(iface + 1);
         len -= strlen(iface);
 
         if (*netif == NULL) {