Make the CGI binary specified explicitly

The CGI binary is now required like the normal PHP binary, and is set by Pyrus's "php_cgi_bin" setting.

The current handling where only the CGI bin from the PATH environment variable is respected doesn't really work for some reason, and even if it did, it's not a flexible option.

This PR also adds support for FILE_EXTERNAL and FILEEOF.

(Requires pyrus/Pyrus#136 to function properly)