FPM support for S6 service startup notifications

Description

S6 and specifically its overlay is a popular set of programs for process supervision. It is very light and often used to run PHP-FPM with other services inside a single container (e.g. Docker). Currently to notify it about service startup, it needs to use s6-notifyoncheck program that runs as FPM master subprocess that polls FPM. It's not optimal and it has got its own issue described in #10315. S6 offers its own notification mechanism that would be better.

The idea is that FPM would receive the descriptor number when started and when it's ready it would write new line to that descriptor which would notify S6 that it's started.

まず S6 notifywhenup のドキュメントとリンク先の s6-overlay のコンテキストを読み、既存の FPM の起動動作と比較します。現在のポーリング方式については issue #10315 を確認します。FPM が起動時にディスクリプタを受け取り、準備が完了した時点で readiness 行を書き込めるようになり、その動作が関連する FPM テストで検証されれば完了です。