BibExport: update Google Scholar exporter

* Use standard sitemap output instead of NLM

* Updated sitemap export method to explicitly exclude
  restricted records.

* Generate alternate URLs for homepage. Consequently fixes the
  dropping of "ln" parameter.

Signed-off-by: Jerome Caffaro <jerome.caffaro@cern.ch>
Tested-by: Tibor Simko <tibor.simko@cern.ch>