FastCGI socket owner, group and mode change support

Adds three new hdf / ini settings: `Server.FileSocketOwner` / `hhvm.server.file_socket_owner`, `Server.FileSocketGroup` / `hhvm.server.file_socket_group`, `Server.FileSocketMode` / `hhvm.server.file_socket_mode` which allows to change owner, group and permissions for UNIX socket file used by FastCGI server. Should work similar as php-fpm `listen.owner`, `listen.group`, `listen.mode`. This allows running hhvm and front-end webserver (e.g. nginx) under different users through UNIX sockets.

Also added is support of specifying octal values for integer settings in php.ini.

P.S. php-fpm defaults to mode 0660, HHVM defaults to 0650. Shouldn't we also change that?