Werk #7068: Truncate values of the notification context which are too long

If the the value stored in the notification context was larger than the OS limit MAX_ARG_STRLEN a failed notification was produced and the error 'OSError: [Errno 7] Argument list too long' appeared in the notify.log. This was usually the case for very long values in the long outputs of a service like e.g. fileinfo.groups. Now the values are truncated and the text 'Attention: Removed remaining content because it was too long.' is shown at the end.

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