Add "hhvm.server.ip" ini setting, matching "Server.IP" HDF setting

For exemple, this allows to specify which IP address the FastCGI server will listen on. On a dual-IP-stack system (IPv4+IPv6), HHVM would listen on `::` by default (IPv6). With this setting, one can make it listen to `0.0.0.0` instead (IPv4) using a php.ini file, instead of an HDF configuration file.