#436 Support POSIX timers and clone pidfds

POSIX timer syscalls return ENOSYS. Add support for up to 64 realtime
or monotonic timers with signal or silent notification. Use the
existing signal queues for delivery and overrun counts. Fork does
not inherit timers, and exec removes them.

clone accepts CLONE_PIDFD but leaves the output pointer untouched.
Return a close-on-exec pidfd before releasing the child, and cancel
the child if allocation or copyout fails. Pidfd creation remains
unsupported for threads and in-process VM clones.

AI disclosure: OpenAI Codex was used to write and review this change.