Detect Systemd presence for php-fpm on *nix systems

[Pierstoval]

When running on Docker or some specific distros, we can have such output when starting the server:

web_1  |  2021-01-25T22:16:13.772Z  INFO > Starting PHP...
web_1  |  2021-01-25T22:16:13.814Z  INFO > Using php-fpm
web_1  |  2021-01-25T22:16:13.815Z  INFO > Running php-fpm with PID 10
web_1  | [25-Jan-2021 22:16:13] ERROR: [/root/.rymfony/fpm-conf.ini:12] unknown entry 'systemd_interval'
web_1  | [25-Jan-2021 22:16:13] ERROR: failed to load configuration file '/root/.rymfony/fpm-conf.ini'
web_1  | [25-Jan-2021 22:16:13] ERROR: FPM initialization failed
web_1  | thread 'main' panicked at 'PHP server exited with exit code: 78', src/php/php_server.rs:50:29
web_1  | note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

The systemd_interval value should be dropped when running a system that isn't started with systemd.

Across some quick research, it seems that this command might help:

ps --no-headers -o comm 1

It either returns init or systemd, and might help detecting this.

There are other ways, such as running systemctl to see if it's available or not, or check if file /sbin/init shows a symbolic link or an executable.

I'm not sure which one should be used by Rymfony, but what I'm sure is that this should be a one-time system check, cached somewhere in Rymfony's global configuration.

[Pierstoval]

Fixed in v0.1.1 (see changelog)