cli.class.php doesn't import/export custom fields with multi-select values

API calls on custom fields with multi-select values (including checkboxes) return an array. cli.class.php and bin/csv/export.php simply return "Array" when exporting such a value, and likewise don't know how to import those values.

This patch implodes those arrays on export and inserts the \0x01 control character between the values (similarly to how they're stored in MySQL). It also explodes fields that contain \0x01 into arrays for import. This allows import/export of custom fields with multi-select values.