Add retain basic implementation on GNU/Linux

Retain is saved in file and backup of this file is done every time.
File isn't saved on every PLC cycle to avoid high I/O load.
By default, retain is saved every second and on PLC stop/shutdown.
It's possible to override retain flush period by using CFLAGS option
in project's settings. For example, for 0.5 seconds period
use "-DFILE_RETAIN_SAVE_PERIOD_S=0.5".

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Beremiz's user manual

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Contents:

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.. toctree::

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   :maxdepth: 2

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   install

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   start

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   edit

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   build

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   connectors

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   debug