Runtime: Fix bugs in Linux target.

- next occurence absolute time computation failing on 32-bit because of casting
- PLC thread was started before period was set, potentially passing uninitialized timespec to clock_nanosleep()

Beremiz's user manual
=====================

Contents:

.. toctree::
   :maxdepth: 2

   install
   start
   edit
   build
   connectors
   debug