We need to call waitpid until no more waitable children are available.
This is necessary because SIGCHLD signals may coalesce into one when
multiple children terminate almost simultaneously.
Also, switch to EventLoop's asynchronous signal handling mechanism,
which allows more complex operations in the signal handler.
* In some cases, we can first call sigaction()/signal(), then *not* pledge
sigaction.
* In other cases, we pledge sigaction at first, call sigaction()/signal()
second, then pledge again, this time without sigaction.
* In yet other cases, we keep the sigaction pledge. I suppose these could all be
migrated to drop it or not pledge it at all, if somebody is interested in
doing that.