util-lib: move things that parse ifnames to shared/

In subsequent commits, calls to if_nametoindex() will be replaced by a wrapper
that falls back to alternative name resolution over netlink. netlink support
requires libsystemd (for sd-netlink), and we don't want to add any functions
that require netlink in basic/. So stuff that calls if_nametoindex() for user
supplied interface names, and everything that depends on that, needs to be
moved.

src/shared/socket-netlink.h

@@ -0,0 +1,17 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+#pragma once
+
+#include "in-addr-util.h"
+#include "macro.h"
+#include "socket-util.h"
+
+int make_socket_fd(int log_level, const char* address, int type, int flags);
+
+int socket_address_parse(SocketAddress *a, const char *s);
+int socket_address_parse_and_warn(SocketAddress *a, const char *s);
+int socket_address_parse_netlink(SocketAddress *a, const char *s);
+
+bool socket_address_is(const SocketAddress *a, const char *s, int type);
+bool socket_address_is_netlink(const SocketAddress *a, const char *s);
+
+int in_addr_ifindex_from_string_auto(const char *s, int *family, union in_addr_union *ret, int *ifindex);