Use the same retain implementation for Win32 targets

Currently plc_Win32_main_retain.c is exact copy of
plc_Linux_main_retain.c, because it's not possible to use symbol
links.

It'd be nice in the future to have possibility to modify targets.code.

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".

Add helper function GetRetainSize()

it will be useful for retain implementations.

use CLOCK_MONOTONIC instead of CLOCK_REALTIME for timer setup

CLOCK_REALTIME is a wall clock, that could change back and forth (user
changes system time, winter/summer time, NTP corrections and so on).
For real-time application CLOCK_MONOTONIC should be used, it doesn't
have such jumps.

Add function example to first_steps example

It'll be used in documentation to show how to create function in
Beremiz.

fix type introduced by f33942053466 (fix pylint warning "(undefined-loop-variable) Using possibly undefined loop variable 'X'")