MDL-38194 Files: Allow download of large backup files

The PHP readfile function does not work for files more than 2GB.
This commit provides a wrapper that can be used so that files less
than 2GB are sent with readfile, and files larger than 2GB are
sent manually.

The change applies to all uses of pluginfile.php, although in
reality, backup files are probably the only ones likely to be more
than two gigabytes.