php: fpm sapi: install php-fpm.conf

Install a custom php-fpm.conf instead of the stock one.

- Listening on /var/run/php-fpm.sock instead of TCP port so we can
  restrict access to webserver user www-data.
- Using ondemand pm, so PHP children are only started after a PHP
  request, instead of kept resident all the time.

Signed-off-by: Floris Bos <bos@je-eigen-domein.nl>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>