add clear_env option to FPM config

This makes it possible to leave the envoronment as is on startup and
pass all the variables to the workers.
The default value of clear_env is "yes", preserving previous behaviour.
Patch by Paul Annesley.
add news entry
Merge branch 'PHP-5.4' into PHP-5.5

* PHP-5.4:
  add news entry
  add clear_env option to FPM config