This commit lands full ECDSA key support when using the OpenSSL
backend. Which includes:
New KEX methods:
ecdsa-sha2-nistp256, ecdsa-sha2-nistp384, ecdsa-sha2-nistp521
Can now read OpenSSL formatted ECDSA key files.
Now supports known host keys of type ecdsa-sha2-nistp256.
New curve types:
NID_X9_62_prime256v1, NID_secp384r1, NID_secp521r1
Default host key preferred ordering is now nistp256, nistp384,
nistp521, rsa, dss.
Ref: https://github.com/libssh2/libssh2/issues/41
Closes https://github.com/libssh2/libssh2/pull/206