src/tripe-ifup.in: Hack IPv6 up on VPN interfaces.

It seems that some devices try to turn off IPv6 on all interfaces if
they can't figure out a route.  But we know how to take care of
ourselves, and we want IPv6 over our VPN even if we can't do it
globally.

I'm thinking specifically of ConnMan on Jolla phones here, but there are
probably others.