Add options for choosing domain to Views relationships between Users and Contacts

With a multi-site CiviCRM instance, that is more than 1 Drupal site hooked up to the same CiviCRM database, it is possible and likely for the same CIviCRM contact to be linked to multiple Drupal users, one per domain.
In this case, a contact will have multiple uf_match records, with different domain_id values.

The Views relationships that connect Drupal users to contacts, and contacts to users, may need an option to add a condition to the join connecting users to uf_match to contacts tables.

What would be nice is 2 things:
1) A checkbox, "Use Current Domain" that would divine the civicrm_domain machine id, and add a condition to the join
2) A select list, for the user to select which domain id they want to use as a condition on the join.