New API method: WebGUI::Response::sendFile; it, as appropriate, calls
setRedirect or setStreamedFile, depending on enableStreamingUploads config var.
setStreamedFile now kicks off the XSendfile process.
File.pm now uses this instead of trying to set both a redirect and a stream.
IO::File::WithPath blows up if a file doesn't exist so this raises an exception
now.
The http now no longer insist that '0' is not a valid filename to stream.
site.psgi, depending on enableStreamingUploads, enables either the Static
or XSendfile middleware.
