Werk #6410: Determine the parent process more reliably

Previously, the parent process was determined by parsing /proc/{PID}/stat and columns where assumed to be separated by space. This was unreliable because the filename in the second column may contain a space as well. If omd was issued e.g. from tmux the process name "tmux: server" was used. This resulted in a traceback during e.g. omd rm. Now the psutil module is used to determine the parent process.

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