spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

if_ovpn: notify userspace when we've used half of the sequence numbers

OpenVPN uses the sequence number (as well as a userspace supplied nonce)
to build the IV. This means we should avoid re-using sequence numbers.
However, userspace doesn't know how many packets we've sent (and thus
what sequence number we're up to).

Notify userspace when we've used half of the available sequence numbers
to tell it that it's time for a key renegotiaton.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D39570

if_ovpn: implement OVPN_GET_PEER_STATS

Allow userspace to retrieve per-peer traffic stats.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D37604

if_ovpn: remove OVPN_SEND_PKT

OpenVPN userspace no longer uses the ioctl interface to send control
packets. It instead uses the socket directly.
The use of OVPN_SEND_PKT was never released, so we can remove this
without worrying about compatibility.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D37602

if_ovpn: extend notifications with a reason

Extend peer deleted notifications (which are the only type right now) to
include the reason the peer was deleted. This can be either because
userspace requested it, or because the peer timed out.

Reviewed by: zlei
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D37583

if_ovpn(4): implement ioctl() to set if_flags

Fully working openvpn(8) --iroute support needs real subnet config
on ovpn(4) interfaces (IFF_BROADCAST), while client-side/p2p
configs need IFF_POINTOPOINT setting. So make this configurable.

Reviewed by: kp

ovpn: Introduce OpenVPN DCO support

OpenVPN Data Channel Offload (DCO) moves OpenVPN data plane processing
(i.e. tunneling and cryptography) into the kernel, rather than using tap
devices.
This avoids significant copying and context switching overhead between
kernel and user space and improves OpenVPN throughput.

In my test setup throughput improved from around 660Mbit/s to around
2Gbit/s.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D34340