[xslt] threaded use with xsltCleanupGlobals()

["Sam Liddicott" <sam liddicott com>]

I think that a multi-threaded user of libxslt should generally only call
xsltCleanupGlobals() when the application is about to shut down;

What nasty thing might happen if a thread calls xsltCleanupGlobals() and
maybe while another thread is doing xslt processing?

(And the same for xmlCleanupParser I guess).

I guess it is ok to call xsltFreeStylesheet and xmlFreeDoc from a thread
as a thread-private reference is passed.

Sam