Issue: https://github.com/php/php-src/issues/10316
Author: bukka
### Description
[S6](https://skarnet.org/software/s6/) and specifically its
[overlay](https://github.com/just-containers/s6-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](https://skarnet.org/software/s6/s6-notifyoncheck.html) 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](https://skarnet.org/software/s6/notifywhenup.html) 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.