dhcpv6-pd: T2741: support delegation on non existing interfaces
dhcpv6-pd: T2741: support delegation on non existing interfaces

We must ignore any return code when invoking dhcpc6 initially. This is required
to enable DHCPv6-PD for interfaces which are yet not up and running and my be
started later by VyOS.

vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x