Turn SSH-2 MACs into a classoid.

This piece of tidying-up has come out particularly well in terms of
saving tedious repetition and boilerplate. I've managed to remove
three pointless methods from every MAC implementation by means of
writing them once centrally in terms of the implementation-specific
methods; another method (hmacmd5_sink) vanished because I was able to
make the interface type 'ssh2_mac' be directly usable as a BinarySink
by way of a new delegation system; and because all the method
implementations can now find their own vtable, I was even able to
merge a lot of keying and output functions that had previously
differed only in length parameters by having them look up the lengths
in whatever vtable they were passed.

cproxy.c

@@ -19,10 +19,10 @@
 static void hmacmd5_chap(const unsigned char *challenge, int challen,
 			 const char *passwd, unsigned char *response)
 {
-    void *hmacmd5_ctx;
+    struct hmacmd5_context *hmacmd5_ctx;
     int pwlen;
 
-    hmacmd5_ctx = hmacmd5_make_context(NULL);
+    hmacmd5_ctx = hmacmd5_make_context();
 
     pwlen = strlen(passwd);
     if (pwlen>64) {