[simple patch] Expose ISO_STREAM via isolation flag config option

This patch exposes ISO_STREAM as a torrc isolation flag IsolateStream, along with what I think is a very convincing warning in the manpage against general use. So why this option?

1. It's quite useful for managing a pool of long running connections, etc.
2. I remember some project (a part of Tails? Whonix? not sure) rigging up something inferior by generating unique Socks credentials for each request, which is messy and doesn't work for TransPort.
3. Since ISO_STREAM is already used internally, nothing beyond adding the config option was needed for this super simple patch.

Testing with wget check.tpo showed it to work as expected here.