cmd/containerboot: add iptables based MSS clamping for ingress/egress proxies

In typical k8s setups, the MTU configured on the eth0 interfaces is typically 1500 which
results in packets being dropped when they make it to proxy pods as the tailscale0 interface
has a 1280 MTU.

As the primary use of this functionality is TCP, add iptables based MSS clamping to allow
connectivity.

Updates #502

Signed-off-by: Maisem Ali <maisem@tailscale.com>
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