php7-fpm can set its created socket's permission with POSIX ACL.
Now
- php7-fpm packages example config file
(/etc/php7/fpm/php-fpm.d/www.conf.default) contains the possible ACL options
`listen.acl_users` and `listen.acl_groups` - commented out, but they are there
- if we try to set these options php-fpm service will fail to start with
`unknown entry 'listen.acl_users'`. Quick search indicates that php-fpm (and
php?) should have been compiled with ACL support.

Either remove the acl options from the config or (preferrably!) compile the
php-fpm package with switches so it can actually handle the options of the
package's example config.

Thanks!