unix: rework signal handling

There was a recent(ish) PoC where someone exploited a file write to /proc/self/fd/<n> to manipulate libuv's signal pipe. (I lost the link; it's in node's bug tracker somewhere but that place is such a swamp...)

Libuv's signal handler writes pointers to said pipe (something I never liked in the first place) and that's enough to get a jump-off point for further exploitation.

I've wanted for a long time to redo our signal handling and I did a work-in-progress first pass in https://github.com/bnoordhuis/libuv/commit/120cf7c034761d10078779e35b840ba3a6228d7c (tl;dr use atomics and per-signal queues of interested event loops)

If anyone has concerns about that approach, LMK. No ETA unless you want to sponsor me :-)

Start by reviewing the linked work-in-progress commit and libuv's existing signal handler, focusing on the signal pipe, atomics, and per-signal queues of interested event loops. The issue does not define acceptance criteria or name files and tests, so the intended completed behavior needs to be established before implementation.