Wake select and epoll_wait on a queued signal

[xalestar]

Part of #378; futex is left for after #383.

A thread parked in a host wait learns of a guest signal only through the wakeup pipe. pselect6 joined the pipe for an indefinite wait only, and epoll_pwait never did, so the handler ran only once the current 200 ms slice ended.

pselect6 now joins the pipe for every wait except a zero timeout, which does not park and would only drain a wake meant for a sibling. epoll_pwait parks in poll() on the kqueue fd plus the pipe, then collects with a zero-timeout kevent(), so the pipe is never registered on the guest's epoll instance.

Reproduction: tests/test-wait-signal-latency.c parks a sibling thread in each wait on an empty pipe, sends SIGUSR1 with pthread_kill, and takes the median signal-to-handler gap over 8 rounds against a 20 ms bound. On 0a63368 all three cases fail at about 42 ms (select with a timeout, epoll_wait with and without one); with this change all three pass.

Validation, rebased on 0a63368:

• make check: exit 0
• test-wait-signal-latency, test-wait-process-signal, test-wait-sigmask-signal, test-nanosleep-signal-latency: pass
• make test-matrix-elfuse-aarch64: 291 passed, 1 failed, 10 skipped. The failure is test-sigio (no SIGURG on a TCP OOB byte). It also fails 3/3 on 0a63368 and does not touch select or epoll.
• The qemu lane was not run.

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Summary by cubic

Wakes threads parked in select/pselect6 and epoll_wait/epoll_pwait immediately when a guest signal is queued, instead of letting the handler wait out the rest of the 200 ms slice.

• pselect6 now joins the wakeup pipe on every wait except a zero-timeout poll, which does not park.
• epoll_pwait now parks in poll() on the kqueue fd plus the pipe, then collects with a zero-timeout kevent(), so the pipe never shows up on the guest's epoll instance.
• Adds tests/test-wait-signal-latency.c to measure signal-to-handler latency for finite and indefinite waits.
• The futex case is left for a separate change (part of A guest signal does not interrupt futex, epoll_wait or a finite select #378).

^{Written for commit 3b9c809. Summary will update on new commits.}

[cubic-dev-ai]

Review completed against the latest diff

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