if_ovpn: pass control packets through the socket

Rather than passing control packets through the ioctl interface allow
them to pass through the normal UDP socket flow.
This simplifies both kernel and userspace, and matches the approach
taken (or the one that will be taken) on the Linux side of things.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D37317