Set `net.ipv4.conf.all.forwarding=1` and `net.ipv6.conf.all.forwarding=1` in network namespace

popd is not setting the sysctl net.ipv4.conf.all.forwarding=1 and net.ipv6.conf.all.forwarding=1 settings, which prevents traffic from being forwarded.

See https://git.ampr.org/adam/44net_Dashboard/-/blob/main/src/pop/interfaces/interfaces.py?ref_type=heads#L34 for the previous Python implementation.