API to support ECDHE through OpenSSL

[conz27]

I'm opening this issue due to address a shortfall of ASIO when it comes to Elliptic Curve Cryptography (ECC) support. Currently, there is no way to initialize the Diffie-Hellman parameters required to support cipher suites that leverage ECDHE.

For regular DHE, there are functions use_tmp_dh() and use_tmp_dh_file(), but those will not work for ECDHE due to the different mechanism required to set the parameters.

I have a hack fix that I can try to turn into an API for ASIO, but I don't want to go through the effort if it's not something that is determined as needed by the maintainers of ASIO.