Make PHP-FPM proctitle customisable

[WilliamDEdwards]

Description

AFAICS, PHP-FPM's proctitle is set to php-fpm + version (e.g. php-fpm8.3).

On machines running multiple PHP-FPM processes (i.e. multiple master processes, not a single process with multiple pools), it would be useful to have separate proctitles. E.g. for tools such as atop that group total resource consumption per procname.

This could be achieved either by making the proctitle customisable, or adding a unique per-process identifier (can't think of one though).

[bukka]

Seems like reasonable FR. Can't really promise how quickly this gets implemented as there are lots of other items on my TODO list. The quickest way would be if someone else creates the PR.

I think we would probably need some config option to provide some identifier or maybe some sort of option to enable adding a master pid to proc title. Or maybe some format option for the whole proc title like we have for access log but not sure if it's worth it. Should be opt it for sure though.