MFC 323728,323729

323728
hyperv/hn: Fix MTU setting

- Add size of an ethernet header to the value configured to NVS. This
does not seem to have any effects if MTU is 1500, but fix hypervisor
side's setting if MTU > 1500.
- Override the MTU setting according to the view from the hypervisor
side.

Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D12352

323729
hyperv/hn: Incease max supported MTU

Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D12365

MFC 322488

hyperv: Update copyright for the files changed in 2017

Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D11982

MFC 316520

hyperv/hn: Fixat RNDIS rxfilter after the successful RNDIS init.

Under certain conditions on certain versions of Hyper-V, the RNDIS
rxfilter is _not_ zero on the hypervisor side after the successful
RNDIS initialization, which breaks the assumption of any following
code (well, it breaks the RNDIS API contract actually). Clear the
RNDIS rxfilter explicitly, drain packets sneaking through, and drain
the interrupt taskqueues scheduled due to the stealth packets.

Reported by: dexuan@
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D10230

MFC 308018,308116

308018
hyeprv/hn: Rename cleaned up RNDIS header file.

Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D8360

308116
hyperv/hn: Rename cleaned up RNDIS source file.

Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D8361