implement php-fpm process-manager 'ondemand', fixes #1035

Signed-off-by: Michael Kaufmann (d00p) <d00p@froxlor.org>
php-fpm sanity checks if pm='dynamic' or pm='ondemand', fixes #955

Signed-off-by: Michael Kaufmann (d00p) <d00p@froxlor.org>