FPM: reverting master process back to root

Leveraging work in parent...

Removing /dev/stdout hacks that a completely unprivileged php-fpm work. This is deemed an unnecessary precaution for its master process, which is then further mitigated with Docker's user namespaces going forward.

Works around:
moby/moby#31243
moby/moby#31106

Container is now verified to run in Docker 1.12+ with user namespaces enabled and selinux in enforcing mode