Some operations like interface creation may need to return metadata
- in this case, interface name - back to the caller if the operation
is successful.
This change implements attaching an `NLMSGERR_ATTR_COOKIE` nla to the
operation reply message via `nlmsg_report_cookie()`.
Additionally, on successful interface creation, interface index and
interface name are returned in the `IFLA_NEW_IFINDEX` and `IFLA_IFNAME
TLVs, encapsulated in the `NLMSGERR_ATTR_COOKIE`.
Reviewed By: pauamma
Differential Revision: https://reviews.freebsd.org/D38283
MFC after: 1 week