ssl: fix elliptic curve selection in server mode

The server code erroneously took the list of curves supported by the
client from it's own hello extension, effectively breaking curve
selection all together.

Also the default fallback secp256k1 curve is not supported by
all clients. secp256r1 is recommended as part of the NIST Suite B
cryptographic suites. The chances are much better that all clients
support it, so use that as fallback.