<OMD_ROOT>/etc/auth.secret is programmatically generated on the first login
to a site or by an omd update. omd diff is only meant for static,
user-modifiable files and therefore reports local modifications to this file on
every site that has been logged into at least once. This change removes said
file from omd diff output to avoid this confusing output. Another effect of
this change is, that the file permissions are now handled by the site directly,
which leads to more restrictive permissions of 0600 instead of 0660.
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