util/linuxfw: add nftables support

This commit adds nftable rule injection for tailscaled. If tailscaled is
started with envknob TS_DEBUG_USE_NETLINK_NFTABLES = true, the router
will use nftables to manage firewall rules.

Updates: #391

Signed-off-by: KevinLiang10 <kevinliang@tailscale.com>
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