(can_* == can_read, can_write, and can_create)
See top of contrib/adc/adc.common-functions for more on this.
Note: the old style (calling get_rights_and_owner with $repo, then
checking $perm_read, $perm_write, etc.), will still work fine.
(we quietly do not document the 'able' adc, which is now the most
"official" adc in the sense that it has a new test, t64-write-able!)
other notes: fix bug in 'able' (not setting $loc)