Now new_connection() takes an optional LogPolicy * argument, and
passes it on to the SshProxy setup. This means that SshProxy's
implementation of the LogPolicy trait can answer queries like
askappend() and logging_error() by passing them on to the same
LogPolicy used by the main backend.
Not all callers of new_connection have a LogPolicy, so we still have
to fall back to the previous conservative default behaviour if
SshProxy doesn't have a LogPolicy it can ask.
The main backend implementations didn't _quite_ have access to a
LogPolicy already, but they do have a LogContext, which has a
LogPolicy vtable pointer inside it; so I've added a query function
log_get_policy() which allows them to extract that pointer to pass to
new_connection.
This is the first step of fixing the non-interactivity limitations of
SshProxy. But it's also the easiest step: the next ones will be more
involved.
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