Werk #13865: Agent controller on Unix: Deny registration if agent socket is not operational

Upon startup, the agent controller (cmk-agent-ctl) now checks if the agent socket is operational on Unix systems. If not, we refuse the agent registration (subcommands register, import), because a non- operational socket can hint at xinetd systems. On such systems, a successful registration would mislead the user into believing that the agent communication is now TLS-encrypted, which is not the case.

Furthermore, the operational status of the agent socket is also reported by the status subcommand.

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