Instead, grab our source code, send it over the link, and have python eval
it and then start the server.py main() function.
Strangely, there's now *less* horrible stuff in ssh.py, because we no longer
have to munge around with the PATH environment variable. And this
significantly reduces the setup required to get sshuttle going.
Based on a suggestion from Wayne Scott.
It seems ssh is kind of stupid and uses a really big SO_SNDBUF (hundreds of
kbytes). Thus, we can't depend on the socket's output buffer to limit our
latency down to something reasonable. Instead, limit the amount of data we
can send in a single round trip.
