Eliminated usage of xsltGetNsProp() in cases where an attribute with a

Eliminated usage of xsltGetNsProp() in cases where an attribute with a

* libxslt/xslt.c libxslt/xsltutils.c libxslt/preproc.c
  libxslt/namespaces.c libxslt/imports.c
  libxslt/attributes.c: Eliminated usage of xsltGetNsProp() in cases
  where an attribute with a specific namespace is requested.
  xsltGetNsProp() uses xmlGetProp() which is not namespace aware
  and thus will return the first attribute with the requested
  name but of arbitrary namespace.
  Changed retrieval of the attributes "exclude-result-prefixes"
  and "extension-element-prefixes", which are expected to be in
  no namespace on XSLT elements and in the XSLT namespace on
  literal result elements or extension elements.
  Additional change: for XSLT elements the attribute
  "exclude-result-prefixes" is only allowed on xsl:stylesheet
  and xsl:transform. This attribute was previously processed on
  all XSLT elements.