BibFormat: advertise /doi URL in DC output

• In Dublin Core output (DC + OAI_DC), prefer the CFG_SITE_RECORD/doi/[...] url scheme over the CFG_SITE_RECORD/record/[...] one.

• Introduces CFG_BIBUPLOAD_INTERNAL_DOI_PREFIX to differentiate between internally managed DOIs, and DOIs assigned/owned by others.

Signed-off-by: Jerome Caffaro <jerome.caffaro@cern.ch>
Tested-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>