reaper: add optional callback

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.

spawn.c

@@ -55,7 +55,7 @@ spawn(struct reaper *reaper, const char *cwd, char *const argv[],
     close(pipe_fds[0]);
 
     if (ret == 0) {
-        reaper_add(reaper, pid);
+        reaper_add(reaper, pid, NULL, NULL);
         return true;
     } else if (ret < 0) {
         LOG_ERRNO("failed to read from pipe");