audio: handle reinit after AO reload slightly cleaner

Don't print bogus messages about packets read in verbose mode.
audio/out: make ao_request_reload() idempotent

This is what you would expect. Before this commit, each
ao_request_reload() call would just queue a reload command, and then
recreate the AO for the number of times the function was called.

Instead of sending a command, introduce some sort of event retrieval
mechanism. At least for the reload case, use atomics, because we're too
lazy to setup an extra mutex.
video/out: minor simplification to event query function

The "clear" parameter is confusing and useless.
command: send property-change event on playlist change