improved KDB logic

[markus2330]

• kdbGet should never fail by default (to allow admins to fix anything) allow kdbSet even after failed kdbGet #2955
• on request programs should be able to fail kdbGet, maybe using proposal: kdbEnsure #868
• should check if system/elektra is updated and remount everything if needed
• get everything (mmap) so that all relevant keys are available for links

[stale]

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