tls: set ecdhCurve default to 'auto'

For best out-of-the-box compatibility there should not be one default
`ecdhCurve` for the tls client, OpenSSL should choose them
automatically.

See https://wiki.openssl.org/index.php/Manual:SSL_CTX_set1_curves(3)

PR-URL: https://github.com/nodejs/node/pull/16853
Refs: https://github.com/nodejs/node/issues/16196
Refs: https://github.com/nodejs/node/issues/1495
Refs: https://github.com/nodejs/node/pull/15206


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>