macOS does not support `SO_PRIORITY` on sockets, but it does apparently
support configuring sockets with a priority flag via a parameter called
`SO_NET_SERVICE_TYPE`. It doesn't appear to be especially well
documented, but it seems to exist as far back as 10.11 (El Capitan).
This patch sets QoSSession to treat connections as
"responsive multimedia audio/video", which some docs appear to describe
as "low delay tolerant, low-medium, loss tolerant, elastic flow,
variable packet interval, rate and size".
SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.