Werk #14684: Fixed handling of Livestatus socket specification in livedump/cmc

There were a few tiny bugs plus some surprising behavior when specifying the Livestatus socket for livedump/cmcdump. With this change, the rules are simple and consistent now: If there is a -s/--socket commandline option, use that. Otherwise derive the UNIX socket from the OMD_ROOT environment variable if that is set (probably the common case). And finally: If there is no -s/--socket option and no OMD_ROOT, complain and exit.

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