Slow FileManager downloads

The 1024 read length seemed to be causing slow downloads (3 Mbps) on local networks. Changing it to 1000000 like the "Upload and Download" module fixes this problem and I get more than 50 Mbps.  I tried 1,000,000,000 but it locked up my server briefly.

This update matches the value in:
https://github.com/webmin/webmin/blob/master/updown/fetch.cgi