BibRecord: wider use of glob/loc field positions

• For several methods (record_delete_subfield_from, record_add_subfield_into, record_modify_controlfield, record_modify_subfield, record_move_subfield), both global and local field positions are now valid arguments. Please note that the signature of these functions has changed. Client code has been updated.

• Several new position-oriented functions are now available: record_get_field, record_replace_field and record_get_subfields.