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/activate/activate.c

@@ -17,6 +17,7 @@
 #include "pretty-print.h"
 #include "process-util.h"
 #include "signal-util.h"
+#include "socket-netlink.h"
 #include "socket-util.h"
 #include "string-util.h"
 #include "strv.h"