This migrates the key exchange curve group configuration into the OpenSSL
configuration format, which is the only path forward to configure these.
We now prefer a hybrid key exchange for TLS handshake and as a client
we'll send key shares for that and pure X25519, while keeping backwards-
compat for P256 and P384.
The statistics for my personal mail server over the last month show a
clear trend for X25519 key exchanges:
156 secp384r1
225 secp256r1
19541 x25519
a1532a552f
