QSystemNetworkInfo class cannot be used from any other thread than the main thread - will crash otherwise

If QSystemNetworkInfo class is instantiated in a worker thread context and gets destroyed in the same context, crash will occur.

As QSystemNetworkInfoPrivate is global static data, it will get deallocated only when process is shutting down. In otherwords lifetime of QSystemNetworkInfoPrivate object is longer than QSystemNetworkInfo object.
This causes a problem if QSystemNetworkInfoPrivate (or objects it is deallocating) closes / frees Symbian client-server handles in the destructor.

Steps for reproducing the bug:
1. Build and run the attached qsni_test application
2. Wait few seconds and exit the program

Expected behaviour:
Application crashes
Actual behaviour:
Application should not crash

Attached also a call stack of the crash (in the same zip).