Replace more (pointer, length) arg pairs with ptrlen.

The abstract method ssh_key_sign(), and the concrete functions
ssh_rsakex_newkey() and rsa_ssh1_public_blob_len(), now each take a
ptrlen argument in place of a separate pointer and length pair.

Partly that's because I'm generally preferring ptrlens these days and
it keeps argument lists short and tidy-looking, but mostly it's
because it will make those functions easier to wrap in my upcoming
test system.

ssh2userauth.c

@@ -890,7 +890,7 @@ static void ssh2_userauth_process_queue(PacketProtocolLayer *ppl)
                     put_data(sigdata, s->pktout->data + 5,
                              s->pktout->length - 5);
                     sigblob = strbuf_new();
-                    ssh_key_sign(key->key, sigdata->s, sigdata->len, 0,
+                    ssh_key_sign(key->key, ptrlen_from_strbuf(sigdata), 0,
                                  BinarySink_UPCAST(sigblob));
                     strbuf_free(sigdata);
                     ssh2_userauth_add_sigblob(