bibrecord: keep singletons by default

• a recently introduced configuration variable CFG_BIBRECORD_KEEP_SINGLETONS was set to False, thus enforcing removal of singletons during MARCXML parsing (i.e. datafields without subfields were not considered). This is behaviour was making bibupload -c not able anymore to remove fields, and bibdocfile --fix-marc was not letting updating the MARC when the latest document was removed.

• the default is now set to True and all the APIs of bibrecord have been enhanced to make this behaviour configurable at runtime. (closes #126)