ecdh: enable ecdh_sha2_nistp{384,521} kex methods

Summary:
Based on Dirkjan's original patch series here:

 * https://www.libssh.org/archive/libssh/2015-08/0000029.html

Here the changes are adapted for the current master
branch, and expanded to include libgcrypt support.

Co-Authored-By: Dirkjan Bussink <d.bussink@gmail.com>
Signed-off-by: Jon Simons <jon@jonsimons.org>

Test Plan:
 * Ran pkd tests for libcrypto and libgcrypt builds.
 * Ran client torture_algorithms.c tests for libcrypto and libgcrypt builds.
 * Tested across multiple libgcrypts ("1.6.3" and "1.7.6-beta").

Reviewers: aris, asn

Reviewed By: asn

Tags: #libssh

Differential Revision: https://bugs.libssh.org/D7

include/libssh/crypto.h

@@ -55,6 +55,10 @@ enum ssh_key_exchange_e {
   SSH_KEX_DH_GROUP14_SHA1,
   /* ecdh-sha2-nistp256 */
   SSH_KEX_ECDH_SHA2_NISTP256,
+  /* ecdh-sha2-nistp384 */
+  SSH_KEX_ECDH_SHA2_NISTP384,
+  /* ecdh-sha2-nistp521 */
+  SSH_KEX_ECDH_SHA2_NISTP521,
   /* curve25519-sha256@libssh.org */
   SSH_KEX_CURVE25519_SHA256_LIBSSH_ORG
 };