elisctl queue: Add setting user access per queue

[pasky]

Currently, access settings per queue are controlled only by user change. This is very awkward when I create a new queue and would like to grant access to a group of users (who also have access to other queues, that should remain unaffected).

Nice & easy solution would be to allow specifying list of users to access the queue in the queue change command.

(Another possibility would be a way to add/remove specific queue for a user instead of always specifying the full list, but that'd still be O(N) instead of O(1) for my usecase.)

[KPilnacek]

Implemented in #83