Werk #8155: Fix detection of inactive hosts when using Pages feature of PNP4Nagios

The Pages feature of PNP4Nagios ignores hosts where the performance data is not updated anymore. It does this by looking at the modification time of the directory where the host's RRDs are being stored. Since the Micro Core simply opened the existing XML files for writing when updating them, this modification time never changed and PNP thought that all hosts were dead. This has been fixed by now by always creating a .xml.new file frist and then renaming this to .xml. This also avoids very short times where the file is empty.

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