pidfd for a thread or CLONE_VM child never reports exit #441

A pidfd that clone or clone3 makes with CLONE_PIDFD for a thread or a CLONE_VM child never becomes readable. No monitor runs for a task with no host pid, and neither the thread exit path nor the site that publishes a VM child's vm_exit_status (src/runtime/forkipc.c) calls proc_pidfd_notify_exit. A guest that polls the pidfd for the child's exit waits forever, where Linux reports POLLIN.

A fix would notify at both exit sites, and record an exit that lands before pidfd_set_target, since the child can finish before the parent binds the pidfd to it.

Raised in review of #421, where clone(CLONE_VM|CLONE_PIDFD) became able to make such a pidfd; before it only clone3 could.