Add DHCPv6 probing to dhcp-discover tool

[DL6ER]

What does this implement/fix?

This PR extends the embedded dhcp-discover to also scan for IPv6 routers possibly broadcasting DNS configuration.

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Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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2. I have commented my proposed changes within the code.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

Checklist:

• The code change is tested and works locally.
• I based my code and PRs against the repositories developmental branch.
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Copilot reviewed 1 out of 8 changed files in this pull request and generated no suggestions.

Files not reviewed (7)

• src/capabilities.h: Language not supported
• src/config/dnsmasq_config.c: Language not supported
• src/tools/CMakeLists.txt: Language not supported
• src/tools/dhcp-discover.c: Language not supported
• src/tools/dhcp-discover.h: Language not supported
• src/tools/dhcpv6-discover.h: Language not supported
• test/run.sh: Language not supported

[yubiuser]

Noticed three things:

Sending ICMPv6 packet: Network unreachable
RAs received on docker0: -1
DHCP packets received on docker0: 0
DHCP packets received on end0: 1
RAs received on end0: 0
Speicherzugriffsfehler

1. Docker receiving -1?
2. Order of reported RA and DHCP could be better (grouped)
3. Memory error. (Does not happen on development)

[DL6ER]

1. You are not using the latest version as negative displays are not possible with the latest commit (87a2ad5).
2. Not possible without much overhead. All the scanning threads are running parallel and don't sync, but I will try something.
3. Hmm, interesting, this probably has to do with the interface error "network unreachable". For me, probing docker works.

[DL6ER]

Squashed a few insignificant commits

[yubiuser]

You are not using the latest version as negative displays are not possible with the latest commit

Interesting, because it was a fresh checkout. Maybe one of the builders failed at some point?

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Looks good now. Code review will follow.

Error while sending Router Solicitation on docker0: Network unreachable
No answer on docker0
Received 1 DHCP (IPv4) and 0 RA (IPv6) answers on end0