This is the current proton thread priority implementation by @rbernon rebased for upstream with a few `#ifdef`s added since AFAIK Linux is the only operating system where threads have a unique PID which can be used to set niceness on.
~~I also ran `./tools/make_requests` on https://gitlab.winehq.org/mzent/wine/-/commit/6705d3481be0409f7e971c1d2c7a36... as well and `autoconf` on https://gitlab.winehq.org/mzent/wine/-/commit/d7bafe40c411753662b2ad97148a6c... (which does blow up the line count a bit).~~ A few tiny changes ~~(with the ready variable for example)~~ are in anticipation for Part 2, which also adds Mach thread priorities and recalculates thread priorities on process priority change.
Since this is a rather large MR, I hope the split here is appropriate ~~(with the second part being slightly smaller)~~, but I think logically it makes the most sense here.
-- v10: server: Check wineserver privileges on init with -20 niceness. server: Use setpriority to update thread niceness when safe.