Add an 'extra' pointer field to ssh2_macalg.

Similar to the versions in ssh_cipheralg and ssh_keyalg, this allows a
set of vtables to share function pointers while providing varying
constant data that the shared function can use to vary its behaviour.

As an initial demonstration, I've used this to recombine the four
trivial text_name methods for the HMAC-SHA1 variants. I'm about to use
it for something more sensible, though.

ssh.h

@@ -630,6 +630,9 @@ struct ssh2_macalg {
     const char *(*text_name)(ssh2_mac *);
     const char *name, *etm_name;
     int len, keylen;
+
+    /* Pointer to any extra data used by a particular implementation. */
+    const void *extra;
 };
 
 #define ssh2_mac_new(alg, cipher) ((alg)->new(alg, cipher))