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.

Inizia leggendo la documentazione di S6 notifywhenup e il contesto s6-overlay collegato, quindi confrontali con il comportamento di avvio esistente di FPM. Esamina issue #10315 per l'approccio attuale al polling. Il lavoro è completato quando FPM può ricevere un descrittore all'avvio e scrivere una riga di readiness quando è pronto, con il comportamento verificato dai test FPM pertinenti.