Werk #10434: cmk-update-agent: Completely apply proxy settings as configured in agent updater ruleset

At the agent updater ruleset, you can specify how to cope with proxy configurations that exists on the hosts. By default, all proxy configurations should be ignored completely by cmk-update-agent(.exe). However, the agent updater does an initial HEAD request to it's configured Checkmk-server, that did not respect the ruleset's configuration.

This is fixed with this Werk.

If you experience problems emerging from the agent updater erroneously following a configured proxy and because of this failing to connect to Checkmk-server, you can try to either deactivate your proxy setup and do a manual agent update to a newly baked agent, or you can replace the host's cmk-update-agent(.exe) executable with the one from Checkmk-server, located at ~/share/check_mk/agents/plugins/cmk-update-agent, or ~/share/check_mk/agents/windows/plugins/cmk-update-agent.exe respectively.

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