When Stream.connect() fails, have it just use on_disconnect(). Now there
is a single disconnect cleanup path.
Remove cutpasted DiagLogStream setup/destruction, and move it into the
base class (temporarily), and only manage the lifetime of its underlying
FD via Side.close(). This cures another EBADF failure.
The user@host prefix in new-style OpenSSH messages unfortunately takes
the host part from ~/.ssh/config and friends. There is no way to know
which hostname will appear in this string without parsing the OpenSSH
config, nor which username will appear.
Instead just regex it.
Add SSH stub modes to print the new/old errors and add some simple
tests.
This extends the work done in b9112a9cbb
Alex Willmer