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.

Merge

Modbus: more comments to remember some necessary changes.

Modbus/BACnet : add some comments so that we don't forget some pending optimization.

Modbus: Delete stored configuration when identical to default configuration.

BACnet: prevent saving configuration that is same as default, delete it if so.

BACnet and Modbus : fix configuration storage directory