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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{

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    "url":"ws://127.0.0.1:8888",

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    "realm":"Automation",

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    "ID":"wamptest",

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    "password":"1234567890",

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    "key":"ABCDEFGHIJ"

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}