hv_netvsc.4: mlink to hn(4)/if_hn(4) + tag spdx

dmesg for this device says:

hn0: <Hyper-V Network Interface> on vmbus0

Add links so that man hn and man if_hn work.

MFC after: 3 days
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1377

(cherry picked from commit 83e93d53f67448a7546c32cd44cb145eceb6c8fb)