Portscan improvements

[jayjb]

Proposed changes

This commit brings about some improvements to the portscan module.

1. It allows users to specify a path to the iptables binary via a config setting: portscan.iptables_path
2. It checks if the system running opencanary has iptables.
3. if nftables is found, we use iptables-legacy.

Types of changes

What types of changes does your code introduce to this repository?
Put an x in the boxes that apply

• New feature (non-breaking change which adds functionality)
• Documentation Update

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• Lint and unit tests pass locally with my changes (if applicable)
• I have run pre-commit (pre-commit in the repo)
• I have added necessary documentation (if appropriate)
• Linked to the relevant github issue or github discussion (iptables option not recognized because of move to nftables #296)