fix issue with sometimes wrong return code of ProcessLogger


As a result of wrong return code Beremiz gives folowing traceback:
Traceback (most recent call last):
File "./Beremiz.py", line 850, in OnMenu
getattr(self.CTR, method)()
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 925, in _Build
IECGenRes = self._Generate_SoftPLC()
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 568, in _Generate_SoftPLC
return self._Compile_ST_to_SoftPLC()
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 661, in _Compile_ST_to_SoftPLC
C_files.remove("POUS.c")
ValueError: list.remove(x): x not in list

The problem is that both threads (for reading stdout and stderr) call self.Proc.poll(),
that updates internal returncode field. This call is done without any locking and the first thread gets correct result,
but other gets 0 as retval. If 0 gets thread, that afterwards calls callback finish, then wrong return code is returned
to the parent. Now only the thread with a callback polls for the return code, other thread just checked local value.

Additionally function spin() waits now until all threads finish reading their pipes, so the results are always correct.

IEC 61131-3
===========

    IEC-61131 is a normative document provided by the standards organization IEC
    (International Electrotechnical Commission) and describing a standard for
    implementing programmable controllers.


    The part 3 of this document (commonly named IEC 61131-3) specifies syntax and 
    semantics for programming language for programmable controllers. Beremiz
    implements all the languages described in this document. 


    http://www.iec.eu
        
PLCopen TC6
===========
   
    PLCopen is a vendor- and product-independent worldwide association defining
    international standards for various topics related to control programming.
    For this purpose, PLCopen has 6 technical committees.
   
   
    The goal of the sixth committee (TC6) is to define a standard file format, 
    based on XML, for exchanging programmables controllers programmed using 
    IEC 61131-3 languages. Beremiz uses this file format for saving the PLC
    programs of projects.
   
   
    http://www.plcopen.org