This patch implements the 'curves' keyword on server lines as well as
the 'ssl-default-server-curves' keyword in the global section.
It also add the keyword on the server line in the ssl_curves reg-test.
These keywords allow the configuration of the curves list for a server.
The "curves" and the older "ecdhe" SSL options that can be used to
define a subset of curves than can be used in an SSL handshake were not
tested in a regtest yet.