Stop ppoll calling a live descriptor invalid

[xalestar]

ppoll tells the guest a descriptor is invalid when macOS poll() will not put it on a kqueue: /dev/null, /dev/zero, /dev/urandom, directory descriptors and epoll descriptors are all open and, on Linux, ready.

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Summary by cubic

Stops ppoll on macOS from reporting POLLNVAL for live descriptors, matching Linux readiness for devices, directories, and epoll fds. Old: we propagated host poll()’s POLLNVAL; New: detect host-refused entries once and compute their readiness via select(), keeping POLLNVAL only for invalid fds.

• Detect POLLNVAL-only entries once, drop them from the poll set, and re-evaluate via select() (zero timeout) each loop; a refused entry causes a single immediate restart, and calls without them make no extra syscalls.
• Return readiness for always-ready devices and directories; epoll fds report idle vs ready correctly; do not interfere with other pollable entries; honor timeouts and wakeups; do not spin.
• Preserve POLLNVAL for closed/invalid guest fds; other error paths unchanged.
• Treat write readiness as POLLOUT|POLLWRBAND so guest POLLWRNORM is honored; for fds ≥ FD_SETSIZE, return requested readiness bits to mirror Linux.
• Tests cover devices/directories readiness, epoll idle/ready, closed fd retaining POLLNVAL, blocking wait alongside an epoll fd, and POLLWRNORM on /dev/null.

^{Written for commit f0c25bf. Summary will update on new commits.}