Backport mlx4{en,ib}(4) from ^/head

MFCing other pieces would be very structurally disruptive. This just
brings back the manpages so they can be used by end-users and to ease
future backports.

svn:mergeinfo omitted, in part because this is a direct commit to
^/stable/11.

Sponsored by: Dell EMC Isilon

Revert r316226 (I used the wrong commit message)

Pointyhat to: ngie

MFC r315759,r315761:

r315759 (by gjb):

Add mlx5en(4) to the hardware page. [1]
Belatedly bump copyright years after several changes.

r315761:

Add cxgbe(4), ixl(4), and mlx4en(4) to the hardware release notes