Pass flags from agent sign request to ssh_key_sign.

Now each public-key algorithm gets to indicate what flags it supports,
and the ones it specifies support for may turn up in a call to its
sign() method.

We still don't actually support any flags yet, though.

ssh2kex-server.c

@@ -27,7 +27,7 @@ static strbuf *finalise_and_sign_exhash(struct ssh2_transport_state *s)
     strbuf *sb;
     ssh2transport_finalise_exhash(s);
     sb = strbuf_new();
-    ssh_key_sign(s->hkey, s->exchange_hash, s->kex_alg->hash->hlen,
+    ssh_key_sign(s->hkey, s->exchange_hash, s->kex_alg->hash->hlen, 0,
                  BinarySink_UPCAST(sb));
     return sb;
 }