Add support for HMAC-SHA512.

I saw a post on comp.security.ssh just now where someone had
encountered an SSH server that would _only_ speak that, which makes it
worth bothering to implement.

The totally obvious implementation works, and passes the test cases
from RFC 6234.

test/testsc.c

@@ -339,6 +339,7 @@ VOLATILE_WRAPPED_DEFN(static, size_t, looplimit, (size_t x))
     X(Y, ssh_hmac_sha1_96)                      \
     X(Y, ssh_hmac_sha1_96_buggy)                \
     X(Y, ssh_hmac_sha256)                       \
+    X(Y, ssh_hmac_sha512)                       \
     /* end of list */
 
 #define ALL_MACS(X, Y)                                      \