Use auto-clone device /dev/{tun,tap} as default on FreeBSD/DragonFly

DragonFly BSD doesn't pre-create `/dev/tunX` or `/dev/tapX` devices
anymore since 2019-Jul-31 [0].  So it's better to use the auto-clone
device `/dev/tun` or `/dev/tap` as the default TUN or TAP device.
The TUN/TAP device has the same behavior on DragonFly BSD and FreeBSD.

See also pull request: https://github.com/DragonFlyBSD/DeltaPorts/pull/925

[0] https://github.com/DragonFlyBSD/DragonFlyBSD/commit/f1e9a4fff5aaac2be3a291dbfea94f94755991b8