Reap exited PID-managed app-server children

[copyberry]

Reap exited PID-managed app-server children

Why

An app-server child started by a previous updater can remain as a zombie after
termination. Its PID then still appears active, causing daemon shutdown to wait
until its timeout instead of removing the stale PID file.

What changed

• Poll waitpid with WNOHANG while waiting for a PID-managed process to stop.
• Check process state before enforcing the stop deadline so an exit at the
  deadline can still be observed and cleaned up.

Testing

Added a regression test that stops an untracked child and verifies that shutdown
finishes promptly and removes its PID file.

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