Checkmk Werk 13865: Agent controller: Check agent socket operability on Unix

ID: 13865 Title: Agent controller: Check agent socket operability on Unix Component: Checks & agents Level: 1 Class: New feature Version: 2.2.0i1 Upon startup, the agent controller (<tt>cmk-agent-ctl</tt>) now checks if the agent socket is operational on Unix systems. If not, only the subcommands - <tt>delete</tt> - <tt>delete-all</tt> - <tt>help</tt> - <tt>proxy-register</tt> - <tt>status</tt> are available, all other subcommands will abort. In particular, we refuse the agent registration. A non-operational socket can hint at <tt>xinetd</tt> 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 <tt>status</tt> subcommand.