Fix handling of output streams, including in cases of error.

In SSHKit we do some [weird stuff](https://github.com/capistrano/sshkit/blob/master/lib/sshkit/backends/netssh.rb#L122) with stream handling depending on the packets (sic) we receive back from `Net::SSH`.

I'd like to write a test case script that produces buffered, and unbuffered output, line- and character-wise, on standard error, and standard out to test the correct usage.

With a pathologically badly behaved script, we could find a way to make this work reliably, I am sure.