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/basic/socket-util.c

@@ -21,7 +21,7 @@
 #include "escape.h"
 #include "fd-util.h"
 #include "fileio.h"
-#include "format-util.h"
+#include "format-ifname.h"
 #include "io-util.h"
 #include "log.h"
 #include "memory-util.h"