poll: Write back only revents to userspace, matching Linux behavior

[tanyifeng]

doPoll() was modifying pfd.Events (adding POLLHUP|POLLERR) and writing back the entire pollfd struct. Linux only writes back revents via unsafe_put_user() in do_sys_poll(). The polluted events field broke libevent's poll_del(), causing a busy-loop in tmux (CPU 96.6%).

Fix: use an internal mask for POLLHUP|POLLERR in initReadiness() and pollBlock(); write back only the 2-byte revents field per fd in doPoll().

Test: PollTest.EventsFieldNotModified, PollTest.EventsFieldNotModifiedOnError

[tanyifeng]

Reproduction

# In gVisor (runsc) — CPU spikes to ~95%
docker run -d --name tmux-test --runtime=runsc ubuntu:22.04 sleep 3600
docker exec tmux-test bash -c "apt-get update -qq && apt-get install -y -qq tmux procps >/dev/null 2>&1"
docker exec tmux-test bash -c "tmux new-session -d -s test && sleep 30 && ps aux | grep tmux"
# tmux: 94.7% CPU

# In Linux (runc) — CPU stays at 0%
docker run -d --name tmux-ctrl --runtime=runc ubuntu:22.04 sleep 3600
docker exec tmux-ctrl bash -c "apt-get update -qq && apt-get install -y -qq tmux procps >/dev/null 2>&1"
docker exec tmux-ctrl bash -c "tmux new-session -d -s test && sleep 30 && ps aux | grep tmux"
# tmux: 0.0% CPU