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.

先阅读 S6 notifywhenup 文档和链接的 s6-overlay 上下文，然后将其与现有的 FPM 启动行为进行比较。查看 issue #10315，了解当前的轮询方式。当 FPM 能够在启动时接收一个描述符，并在准备就绪时写入一行 readiness，且相关 FPM 测试验证了该行为时，即视为完成。