In attachment.
This is actually just an excuse to store the patch somewhere and possibly collect opinions as I don't really think this should go in because:

- it's UNIX only
- as such, deciding whether using sendfile() should probably be done silently (no explicit argument)
- on the other hand, I don't think it's safe to decide this automatically because:
    - the input fd should be a regular file and it's not clear how to determine this beforehand 
    - in case of disconnection OSError is raised instead of socket.error (minor backward compatibility issue)