sshproxy: share the caller's LogPolicy.

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.

logging.c

@@ -81,6 +81,11 @@ void logflush(LogContext *ctx)
             fflush(ctx->lgfp);
 }
 
+LogPolicy *log_get_policy(LogContext *ctx)
+{
+    return ctx->lp;
+}
+
 static void logfopen_callback(void *vctx, int mode)
 {
     LogContext *ctx = (LogContext *)vctx;