if_ovpn: use IFT_TUNNEL

IFT_ENC has special behaviour in pf we don't desire, and this also ensures that
for all interface types there is N:1:1 correspondence between if_type:dlt:header len.

Requested by: glebius
MFC after: 1 week

(cherry picked from commit ff9f76a206c80c263050816735d537a151ee2999)