util-lib: add function to resolve "alternative" names

Calls to if_nametoindex() are expected to use resolve_ifname() instead.

src/shared/socket-netlink.h

@@ -1,10 +1,14 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 #pragma once
 
+#include "sd-netlink.h"
+
 #include "in-addr-util.h"
 #include "macro.h"
 #include "socket-util.h"
 
+int resolve_ifname(sd_netlink **rtnl, const char *name);
+
 int make_socket_fd(int log_level, const char* address, int type, int flags);
 
 int socket_address_parse(SocketAddress *a, const char *s);