Add a new service module for PHP FPM.

This adds a service for the PHP FastCGI Process Manager which replaces PHPs FastCGI module in future versions.

The service needs PHP with compiled in FPM, and thus is based on MarcWeber's work in his experimental/php branch, so it might make sense to either cherry-pick and/or modify MarcWeber/nixpkgs@3d45a854eb50a2895807e4cc33c548af8c755de4 or add `--enable-fpm` prior to merging in this commit.