Werk #6834: df: Optionally produce metrics compatible to unix df command

Add an option to exclude space reserved for the root user from the calculation of used space, to produce metrics matching the ouput of the unix df command. By default Check_MK treats space that is reserved for the root user on Linux and Unix as used space. Usually, 5% are being reserved for root when a new filesystem is being created. With this new option in the ruleset "Filesystem (used space and growth)" you can have Check_MK exclude the current amount of reserved but yet unused space from the calculations regarding the used space (absolute and percentage).

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