crypto: simplify using pre-existing keys with ECDH

These changes simplify using ECDH with private keys that are not
dynamically generated with ECDH.generateKeys.

Support for computing the public key corresponding to the given private
key was added. Validity checks to reduce the possibility of computing
a weak or invalid shared secret were also added.

Finally, ECDH.setPublicKey was softly deprecated.

PR-URL: https://github.com/nodejs/node/pull/3511


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Shigeki Ohtsu <ohtsu@iij.ad.jp>