app_queue: Add 'prio' setting to the 'force_longest_waiting_caller' option

app_queue: Add 'prio' setting to the 'force_longest_waiting_caller' option

This adds a 'prio' setting to ensure that call priority is respected across multiple queues.
Using 'yes' could cause high-priority callers to be skipped if a caller
in another queue had a longer wait time, regardless of priority.

Resolves: #1637

UserNote: The 'force_longest_waiting_caller' option now supports a 'prio' setting.
When set to 'prio', calls are offered by priority first, then by wait time.