GTK2: Fixed various issues in get_doc() et al.
* Changes API of pgtk2_get_doc(). The pushv parameter has been
replaced with a pointer to the desired svalue position.
* get_doc() did not take advantage of pgtk2_get_doc() supporting
pushing the result on the svalue stack.
* pgtk2_get_doc() returned an uninitialized pointer
when called with a non-zero pushv.
* pgtk2_get_doc() returned a g_new0()'ed svalue when called with
a zero pushv. This value was not freed later.