tree-wide: make format_ifname() or friends return negative errno on failure

tree-wide: make format_ifname() or friends return negative errno on failure

Also,
- drop unnecessary +1 from buffer size, as IF_NAMESIZE or IFNAMSIZ
  includes the nul at the end.
- format_ifname() does not update buffer on failure,
- introduces format_ifname_alloc(), FORMAT_IFNAME(), and their friends.