Change sensitive strbufs/sgrowarrays to the new _nm version.

The _nm strategy is slower, so I don't want to just change everything
over no matter what its contents. In this pass I've tried to catch
everything that holds the _really_ sensitive things like passwords,
private keys and session keys.

ssh2kex-client.c

@@ -565,7 +565,7 @@ void ssh2kex_coroutine(struct ssh2_transport_state *s, bool *aborted)
             /*
              * Encode this as an mpint.
              */
-            buf = strbuf_new();
+            buf = strbuf_new_nm();
             put_mp_ssh2(buf, s->K);
 
             /*