First step toward _private cleanup: use the psvi field to store the

First step toward _private cleanup: use the psvi field to store the

* libxslt/attributes.c libxslt/keys.h libxslt/preproc.c
  libxslt/transform.c libxslt/variables.c libxslt/xsltutils.c:
  First step toward _private cleanup: use the psvi field to store
  the precompilation informations in the stylesheet nodes.
Daniel