BibFormat: improve bfe_fulltext(_mini)

• Improves the algorithm for aggregating and nicely displaying multiple fulltextes attached to a record (if the description, i.e. $y, is enough to distinguish them don't print the docname).

• If CFG_CERN_SITE, don't display the .ps.gz format in case another format is available.

• If 'Fulltext' is in the description, will use the internationalized version.

• Better usage of the separator argument in general.