Message 217165 - Python tracker

Given the opinions expressed so far I:
- got rid of the "blocksize" parameter
- got rid of the "use_fallback" parameter
- added a "count" parameter
- used os.fstat() to figure out the total file size and passed it directly to sendfile()

I'm attaching socket-sendfile6.patch which includes docs and many new tests.
Hopefully this should be the final one (yet to review though).