gnu: services: Add php-fpm.

* gnu/services/web.scm (<php-fpm-configuration>,
  <php-fpm-process-manager-configuration>): New record types.
  (php-fpm-configuration?,
   php-fpm-process-manager-configuration?,
   php-fpm-service-type,
   nginx-php-location): New procedures.
* doc/guix.texi (Web-Services): Document php-fpm service.
* gnu/tests/web.scm: Add php-fpm system test.

Signed-off-by: Christopher Baines <mail@cbaines.net>