Implement "curve448-sha512" kex, from RFC 8731.

With all the preparation now in place, this is more or less trivial.
We add a new curve setup function in sshecc.c, and an ssh_kex linking
to it; we add the curve parameters to the reference / test code
eccref.py, and use them to generate the list of low-order input values
that should be rejected by the sanity check on the kex output; we add
the standard test vectors from RFC 7748 in cryptsuite.py, and the
low-order values we just generated.

testcrypt.c

@@ -340,6 +340,7 @@ static const ssh_kex *get_ecdh_alg(BinarySource *in)
         const ssh_kex *value;
     } algs[] = {
         {"curve25519", &ssh_ec_kex_curve25519},
+        {"curve448", &ssh_ec_kex_curve448},
         {"nistp256", &ssh_ec_kex_nistp256},
         {"nistp384", &ssh_ec_kex_nistp384},
         {"nistp521", &ssh_ec_kex_nistp521},