netlink: return optional metadata with the operation result.

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