Fix error message when KEXINIT negotiation fails.

By putting the wrong error-type enum value in a ScanKexinitsResult, I
accidentally caused nonsense messages of the form

  Selected key exchange algorithm "foo,bar,baz" does not correspond to any supported algorithm

where "foo,bar,baz" is the full comma-separated list sent by the
server, so it's not even _an_ algorithm as the message suggests.

Now the message is the one it should have been all along:

  Couldn't agree a key exchange algorithm (available: foo,bar,baz)

ssh/transport2.c

@@ -1166,7 +1166,7 @@ static ScanKexinitsResult ssh2_scan_kexinits(
              * Otherwise, any match failure _is_ a fatal error.
              */
             ScanKexinitsResult skr = {
-                .success = false, .error = SKR_UNKNOWN_ID,
+                .success = false, .error = SKR_NO_AGREEMENT,
                 .kind = kexlist_descr[i], .desc = slists[i],
             };
             return skr;