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