Making it possible to define the elliptic curve by name for ECDHE

Diffie–Hellman.

Motivation:

Improving security of SSL transactions that use ECDHE-based ciphers.

Modifications:

Adding a new native operation on OpenSSL that makes it possible to set
the elliptic curve by meely giving its name. An exception is thrown in
case the curve is not found or fails to load.

Result:

One can now setup his/his elliptic curve as can be chosen in the liste
generated by:
openssl ecparam -list_curves
This is done e.g. by calling:
SSLContext.setTmpECDHByCurveName(sslContext.context(), "secp256k1")
Note that secp256k1 is the default value.