Support ECDH key exchange.

REQ_KEY requests have an extra field indicating key exchange version.
If it is present and > 0, the sender supports ECDH. If the receiver also
does, then it will generate a new keypair and sends the public key in a
ANS_KEY request with "ECDH:" prefixed. The ans_key_h() function will
compute the shared secret, which, at the moment,is used as is to set the
cipher and HMAC keys. However, this must be changed to use a proper KDF.
In the future, the ECDH key exchange must also be signed.

src/openssl/ecdh.h

@@ -23,6 +23,7 @@
 #include <openssl/ecdh.h>
 
 #define ECDH_SIZE 67
+#define ECDH_SHARED_SIZE 66
 
 typedef EC_KEY *ecdh_t;