kex: Add sntrup761x25519-sha512@openssh.com.

All of the initial work was done by Simon. Jakub cleaned up the
formatting issues, resolved the padding of bignum to match specs
and be interoperable with OpenSSH (and few more minor details).

Closes: #194.

Signed-off-by: Simon Josefsson <simon@josefsson.org>
Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
Reviewed-by: Sahana Prasad <sahana@redhat.com>

src/wrapper.c

@@ -48,8 +48,9 @@
 #ifdef WITH_GEX
 #include "libssh/dh-gex.h"
 #endif /* WITH_GEX */
-#include "libssh/ecdh.h"
 #include "libssh/curve25519.h"
+#include "libssh/ecdh.h"
+#include "libssh/sntrup761.h"
 
 static struct ssh_hmac_struct ssh_hmac_tab[] = {
   { "hmac-sha1",                     SSH_HMAC_SHA1,          false },
@@ -586,6 +587,11 @@ int crypt_set_algorithms_server(ssh_session session){
     case SSH_KEX_CURVE25519_SHA256_LIBSSH_ORG:
         ssh_server_curve25519_init(session);
         break;
+#endif
+#ifdef HAVE_SNTRUP761
+    case SSH_KEX_SNTRUP761X25519_SHA512_OPENSSH_COM:
+        ssh_server_sntrup761x25519_init(session);
+        break;
 #endif
     default:
         ssh_set_error(session,