Makes it possible for users to make the ingress/egress proxies created by the operator to use nftables by setting PROXY_FIREWALL_MODE=nftables. Internally this makes use of the same iptables/nftables functionality used by tailscaled. Also ensures that the same firewall mode (iptables or nftables) are used by both tailscaled and proxy ingress/egress rules
Signed-off-by: Irbe Krumina <irbe@tailscale.com>