remove PAM dependencies on old packet API

Requires some caching of values, because the PAM code isn't
always called with packet context.
diff --git a/monitor.c b/monitor.c
index 1dcf930..5fa30b2 100644
--- a/monitor.c
+++ b/monitor.c
@@ -991,7 +991,7 @@
 	if (!options.use_pam)
 		fatal("UsePAM not set, but ended up in %s anyway", __func__);
 
-	start_pam(authctxt);
+	start_pam(ssh);
 
 	monitor_permit(mon_dispatch, MONITOR_REQ_PAM_ACCOUNT, 1);
 	if (options.kbd_interactive_authentication)