php: enable php-fpm

- php-fpm is an alternative PHP FastCGI implementation which seems to
be now prefered to php-fcgi
- this patch enable the build of php-fpm and packages it in its own
package
- it's tested with hiawatha on an armv5te target

Signed-off-by: Eric Bénard <eric@eukrea.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>