When calling ‘reaper_add()’, the caller can provide a callback. If
non-NULL, the reaper will call the callback to handle the actual
reaping.
If the callback is NULL, or if it returns false, the reaper reaps the
child process.
Use a signalfd to listen for SIGCHLD signals.
When we receive a SIGCHLD over the signalfd, reap all dead children by
looping over all registered child PIDs and call waitpid(WNOHANG) on
them.
