d/php-fpm.service: harden systemd service

Hardening the systemd service unit makes exploiting potential vulnerabilities much harder, thus improving the security of Debian servers running PHP.

See https://bugs.php.net/72510 and https://github.com/php/php-src/commit/40c4d7f1820df1872a71ab07fd26da45a203e37f

Closes: #1009964