Werk #3286: Fixing possible logging issues when using datasource programs for monitoring

When calling datasource programs for monitoring they are executed as subprocesses of Check_MK. Check_MK as passing all filedescriptors except stdout and stderr to the subprocess. So if a subprocess was doing something strange with them it could happen that this influenced the handling within Check_MK.

This could, for example, confuse the communication between the CMC and the Check_MK keepalive helper processes.

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