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: #1377

Author: concussious

share/man/man4/Makefile

@@ -713,6 +713,8 @@ MLINKS+=gpioths.4 dht11.4
 MLINKS+=gpioths.4 dht22.4
 MLINKS+=gre.4 if_gre.4
 MLINKS+=hpet.4 acpi_hpet.4
+MLINKS+=hv_netvsc.4 hn.4 \
+	hv_netvsc.4 if_hn.4
 MLINKS+=${_hptrr.4} ${_rr232x.4}
 MLINKS+=${_attimer.4} ${_i8254.4}
 MLINKS+=ip.4 rawip.4

share/man/man4/hv_netvsc.4

@@ -1,3 +1,5 @@
+.\"-
+.\" SPDX-License-Identifier: BSD-2-Clause
 .\"
 .\" Copyright (c) 2012 Microsoft Corp.
 .\" All rights reserved.