demux_lavf: let libavformat open HLS streams directly

Fixes opening some streams.

This means the HLS playlist will be opened twice, but that's not much of
a problem, considering it's pretty small, and HLS will make many other
http accesses anyway.
demux_lavf: set stream network options if applicable

Normally, we pass libavformat demuxers a wrapped mpv stream. But in some
cases, such as HLS and RTSP, we let libavformat open the stream itself.
In these cases, set typical network properties like useragent according
to the mpv options.

(We still don't set it for the cases where libavformat opens other
streams on its own, e.g. when opening the companion .sub file for .idx
files - not sure if we maybe should always set these options.)
client API: qthelper: remove commented code

This is already taken care of by Q_DISABLE_COPY().
audio/out/push: reset projected EOF time on new data

Seems like this could theoretically happen in low buffer situations, but
I haven't spotted this behavior in the wild.