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' header file for use by extensions */

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#include "iec_types.h"

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#define LOG_LEVELS 4

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#define LOG_CRITICAL 0

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#define LOG_WARNING 1

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#define LOG_INFO 2

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#define LOG_DEBUG 3

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extern unsigned long long common_ticktime__;

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int LogMessage(uint8_t level, char* buf, uint32_t size);

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long AtomicCompareExchange(long* atomicvar,long compared, long exchange);

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