Add option to disable executing `nft`/`iptables` on startup

[dilluti0n]

Current approach of hardcoding firewall rules into the binary negatively impacts both maintainability and the user experience. If bootstrap excluded, dpibreak can be used for non-HTTPS traffic as well, as generic TCP obscurator. (Though I'm not sure what the use case would be...)

Windows lacks binaries or interfaces like nft/nfqueue, hardcoding is unavoidable for that platform and this would be linux-only feature.

See also: #26

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It is still possible to apply rules beyond the predefined ones to dpibreak without disable option.

Example script for registering a rule to exclude traffic destined for the loopback interface:

#!/bin/bash
table=dpibreak
chain=OUTPUT

dpibreak "$@" &
pid=$!

for _ in $(seq 1 50); do
    if nft list chain inet $table $chain &>/dev/null; then
        nft insert rule inet $table $chain oif "lo" return
        break
    fi
    sleep 0.1
done

wait $pid