reaper: new module, uses a signalfd to wait() on child processes

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.

reaper.h

@@ -0,0 +1,12 @@
+#pragma once
+
+#include <sys/wait.h>
+
+#include "fdm.h"
+
+struct reaper;
+
+struct reaper *reaper_init(struct fdm *fdm);
+void reaper_destroy(struct reaper *reaper);
+
+void reaper_add(struct reaper *reaper, pid_t pid);