Elliptic-curve cryptography support.

This provides support for ECDSA public keys, for both hosts and users,
and also ECDH key exchange. Supported curves are currently just the
three NIST curves required by RFC 5656.

putty.h

@@ -253,6 +253,7 @@ enum {
     KEX_DHGROUP14,
     KEX_DHGEX,
     KEX_RSA,
+    KEX_ECDH,
     KEX_MAX
 };