libcrypto: Implement OpenSSH-compatible AES-GCM ciphers using OpenSSL

The commit also propares the internals throughout the code base
for the inclusion of a new AEAD cipher, because previously, the
source code counted only with chacha20-poly1305 cipher, which
is very specific in many cases.

The SSH_HMAC_AEAD_GCM mac algorithm is not actually used, but the name
needed to be defined so we can match in the algorithms selection per
OpenSSH specification (MACs are ignored in case GCM is select as a cipher [1]).

If the provided OpenSSL does not provide  EVP_aes_128_gcm()  function,
the AES-GCM ciphers will not be compiled in.

[1] https://cvsweb.openbsd.org/src/usr.bin/ssh/PROTOCOL.chacha20poly1305?annotate=HEAD

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

include/libssh/crypto.h

@@ -48,6 +48,9 @@
 
 #define DIGEST_MAX_LEN 64
 
+#define AES_GCM_TAGLEN 16
+#define AES_GCM_IVLEN  12
+
 enum ssh_key_exchange_e {
   /* diffie-hellman-group1-sha1 */
   SSH_KEX_DH_GROUP1_SHA1=1,
@@ -78,7 +81,10 @@ enum ssh_cipher_e {
     SSH_AES256_CBC,
     SSH_AES128_CTR,
     SSH_AES192_CTR,
-    SSH_AES256_CTR
+    SSH_AES256_CTR,
+    SSH_AEAD_AES128_GCM,
+    SSH_AEAD_AES256_GCM,
+    SSH_AEAD_CHACHA20_POLY1305
 };
 
 struct ssh_crypto_struct {

include/libssh/wrapper.h

@@ -47,7 +47,8 @@ enum ssh_hmac_e {
   SSH_HMAC_SHA384,
   SSH_HMAC_SHA512,
   SSH_HMAC_MD5,
-  SSH_HMAC_AEAD_POLY1305
+  SSH_HMAC_AEAD_POLY1305,
+  SSH_HMAC_AEAD_GCM
 };
 
 enum ssh_des_e {