hurd if_index: Explicitly use AF_INET for if index discovery

5bf07e1b ("Linux: Simplify __opensock and fix race condition [BZ #28353]")
made __opensock try NETLINK then UNIX then INET. On the Hurd, only INET
knows about network interfaces, so better actually specify that in
if_index.

(cherry picked from commit 1d3decee)