Fix reuse of xmlInitParser

While xmlCleanupParser() should not be used unless complete control
is insured over the programe making sure libxml2 is not in use anywhere
It should still be usable, and allow a sequence of
    xmlInitParser();
    xmlCleanupParser();
calls if needed, the problem is that the thread key wasn't reallocated
on subsequent xmlinitParser() calls leading to corruption of pthread
keys used by the program.

* threads.c: make sure xmlCleanupParser() reset the pthread_once()
             global variable driving thread key allocation.