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.

Commencez par lire la documentation de S6 notifywhenup et le contexte s6-overlay associé, puis comparez-les au comportement de démarrage actuel de FPM. Consultez l’issue #10315 pour connaître l’approche actuelle de polling. Le travail est considéré comme terminé lorsque FPM peut recevoir un descripteur au démarrage et écrire une ligne de readiness lorsqu’il est prêt, le comportement étant vérifié par les tests FPM pertinents.