Create individual PHP session directories per php-fpm user

Summary:
To improve security for applications not using a custom session
handler, it's better to isolate sessions in a directory only
readable by the current php-fpm pool username.

As such, a security issue with one site allowing to browse files
won't allow to hijack sessions on a site served by another php-fpm pool.

Meanwhile, we reset to the default value in php.ini to allow quick tests
with php -S internal server on development servers.

Ref T417.

Test Plan: Test with www.dereckson.be

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T417

Differential Revision: https://devcentral.nasqueron.org/D1486