Werk #14971: Checkmk agent: cached sections are never invalidated

This is a fix for the Checkmk agents AIX, FreeBSD, Linux, OpenWrt, and Solaris. The Checkmk agent is able to cache some sections. This means sections are only created when there is no cached data available, or the cached data is deemed outdated (i.e. invalidated).

When the cached data is invalidated, additional processes are forked that are tasked with the renewal of this data. It could be the case that any one of these processes terminates or runs forever, leaving behind temporary files in the working directory "MK_VARDIR", and the data was never renewed because the updating process was assumed to still be running. The corresponding services of this section turned STALE.

This problem can be verified by looking for old files named ".cache.new" or outdated files named "*.cache" under "MK_VARDIR".

Any such orphaned files that are older than a specified timeout are now deleted, so that the cache is renewed. In addition, any suspiciously long running update processes are killed, so that a retry can take place.

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