Fix child processes not being reaped with PID 1

Starting with Puma v6.4.1, we observed that killed Puma cluster
workers were never being restarted when the parent was run as PID
1. For example, I issued a `kill 44` and PID 44 remained in the
`defunct` state:

```
git@gitlab-webservice-default-78664bb757-2nxvh:/var/log/gitlab$ ps -ef
UID          PID    PPID  C STIME TTY          TIME CMD
git            1       0  0 Jan09 ?        00:01:39 puma 6.4.1 (tcp://0.0.0.0:8080) [gitlab-puma-worker]
git           23       1  0 Jan09 ?        00:05:46 /usr/local/bin/gitlab-logger /var/log/gitlab
git           41       1  0 Jan09 ?        00:01:55 ruby /srv/gitlab/bin/metrics-server
git           44       1  0 Jan09 ?        00:02:41 [ruby] <defunct>
git           46       1  0 Jan09 ?        00:02:38 puma: cluster worker 1: 1 [gitlab-puma-worker]
git           48       1  0 Jan09 ?        00:02:42 puma: cluster worker 2: 1 [gitlab-puma-worker]
git           49       1  0 Jan09 ?        00:02:41 puma: cluster worker 3: 1 [gitlab-puma-worker]
git         5205       0  0 21:57 pts/0    00:00:00 bash
git         5331    5205  0 22:00 pts/0    00:00:00 ps -ef
```

Further investigation showed that the introduction of
`Process.wait2(-1, Process::WNOHANG)` in puma#3255 never appears to return
anything inside Google Kubernetes Engine running as PID 1.

Previously `Process.wait(w.pid, Process::WNOHANG)` was called on each
known worker PID. puma#3255 changed this behavior to do this only if the
`fork_worker` config parameter were enabled, but it seems that we
should always do this.

Closes puma#3313

lib/puma/cluster.rb

@@ -560,9 +560,11 @@ def wait_workers
       @workers.reject! do |w|
         next false if w.pid.nil?
         begin
-          # When `fork_worker` is enabled, some worker may not be direct children, but grand children.
-          # Because of this they won't be reaped by `Process.wait2(-1)`, so we need to check them individually)
-          if reaped_children.delete(w.pid) || (@options[:fork_worker] && Process.wait(w.pid, Process::WNOHANG))
+          # We may need to check the PID individually because:
+          # 1. On some environments running as PID 1, `Process.wait2(-1)` might not return anything.
+          # 2. When `fork_worker` is enabled, some worker may not be direct children, but grand children.
+          # Because of this they won't be reaped by `Process.wait2(-1)`.
+          if reaped_children.delete(w.pid) || Process.wait(w.pid, Process::WNOHANG)
             true
           else
             w.term if w.term?