Next tiny step of refactoring - mostly bug fixes and cosmetic changes.

* libxslt/attributes.c libxslt/documents.c
  libxslt/extensions.c libxslt/keys.c libxslt/pattern.c
  libxslt/preproc.c libxslt/templates.c
  libxslt/transform.c libxslt/variables.c
  libxslt/xslt.c libxslt/xsltInternals.h:
  Next tiny step of refactoring - mostly bug fixes and
  cosmetic changes.
  Changes outside of the refactored code:
  1) Optimized xsl:attribute if the content consists of
   just 1 text node.
  2) Optimized computation of xsl:key. The keys will now be
   computed for a specific document not until the first call
   of a key() function; here only the keys with the specific
   name used by key() are computed. This means that this
   now avoids computation of all keys for all loaded
   input documents (even if no key() was called on them).
   One exception is the scenario where a key() is used in
   a template's match pattern; in this case all keys are
   computed for a document if there's a chance that
   a "keyed" template could match a node (this could still
   be optimized a bit).