Added $thr->_handle() method

From the (updated) POD:

This private method returns the memory location of the internal thread structure associated with a threads object. For Win32, this is the handle returned by CreateThread; for other platforms, it is the pointer returned by pthread_create.

This method is of no use for general Perl threads programming. Its intent is to provide other thread modules with the capability to access, and possibly manipulate, the underlying thread structure associated with a Perl thread.