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/core/dbus-socket.c

@@ -12,6 +12,7 @@
 #include "parse-util.h"
 #include "path-util.h"
 #include "socket.h"
+#include "socket-netlink.h"
 #include "socket-util.h"
 #include "string-util.h"
 #include "unit.h"