MODBUS: Fix saving of websettings.

Runtime/PLCObject : move call to PreStartPLC right after call to the "start" method of all python extensions, and then just before the start of the python loop executing python_* blocks.

Modbus plugin: reformat code, and add comment.

Modbus plugin: fix runtime periodic timer (don't use POSIX timer)

Modbus and Bacnet websettings : Rename variables and functions to avoid name collisions.

Websettings for modbus and bacnet are now passed to runtime as python files loaded (execfile) at init of PLCObject with the same globals.
Because if this, same names used in previously different modules now colide.

Modbus: fix strange side effect of re-use of condition variables or mutex with Xenomai : without bzero of the associated types, mutex keeps locked.

Modbus: fix client timer invalid arguments.

Modbus add a timer thread, and switch timer to SIGEV_THREAD_ID to follow xenomai posix skin restrictions.

Runtime: Fix typo in PLCObject leading to exception when reporting failure.

Py_ext: use non-derived type when generating C code for handling Python PLC globals.

tests/python : add some STRING PLC Python globals with OnChange.

Runtime: STRING type for Python PLC Globals was making problem with strings, since generated code wasn't doing comparison properly.