Add opac_visible attribute to copy and serial unit statuses

Add opac_visible attribute to copy and serial unit statuses

config.copy_status defines an opac_visible column, but that attribute
was not being exposed in the holdings XML, so code that displayed copies
and serial units based on the holdings XML had to hardcode the display
based on the status ID or name instead of using the configured setting.

To simplify display logic, teach holdings XML about the opac_visible
column and expose its value in the status element as a new opac_visible
attribute.

Signed-off-by: Dan Scott <dscott@laurentian.ca>