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