basic: split ifname related calls from format-util.h into format-ifname.h

This way we don't have to pull in net/if.h into format-util.h.

This is supposed to address https://github.com/systemd/systemd/pull/32212#discussion_r1755639881

No actual code changes, just a .c/.h file split-up.

src/libsystemd-network/network-common.c

@@ -1,7 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
 #include "env-util.h"
-#include "format-util.h"
+#include "format-ifname.h"
 #include "network-common.h"
 #include "socket-util.h"
 #include "unaligned.h"