Enable ECDH cipher support

Fixes issue #984 by enabling OpenSSL to advertise ECDH and ECDSA support in Cherokee. Tested with various clients and Qualys SSL Labs server test.

Similar to the recent Dovecot ECDH patch, I chose default key parameter as NIST P-384 rather than NIST P-256 (default for Apache) to ensure RFC 6460 compliance for AES-256 encryption. Also considered adding a configuration option to select curve but since OpenSSL 1.0.2 automatically selects temporary key parameter, not really needed in short term.