Add SIOCGIFDOWNREASON.

The ioctl(2) is intended to provide more details about the cause of
the down for the link.

Eventually we might define a comprehensive list of codes for the
situations. But interface also allows the driver to provide free-form
null-terminated ASCII string to provide arbitrary non-formalized
information. Sample implementation exists for mlx5(4), where the
string is fetched from firmware controlling the port.

Reviewed by: hselasky, rrs
Sponsored by: Mellanox Technologies
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D21527