Add a key-length field to 'struct ssh_mac'.

The key derivation code has been assuming (though non-critically, as
it happens) that the size of the MAC output is the same as the size of
the MAC key. That isn't even a good assumption for the HMAC family,
due to HMAC-SHA1-96 and also the bug-compatible versions of HMAC-SHA1
that only use 16 bytes of key material; so now we have an explicit
key-length field separate from the MAC-length field.

sshsh256.c

@@ -345,7 +345,7 @@ const struct ssh_mac ssh_hmac_sha256 = {
     hmacsha256_start, hmacsha256_bytes,
     hmacsha256_genresult, hmacsha256_verresult,
     "hmac-sha2-256", "hmac-sha2-256-etm@openssh.com",
-    32,
+    32, 32,
     "HMAC-SHA-256"
 };