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/meson.build

@@ -33,6 +33,7 @@ basic_sources = files(
         'fd-util.c',
         'fileio.c',
         'filesystems.c',
+        'format-ifname.c',
         'format-util.c',
         'fs-util.c',
         'gcrypt-util.c',