Remove dependency of sshrand.c on SHA-512.

Rather like some of the tricks I did in mpint.h, this replaces the
unparametrised function random_setup_special() with one called
random_setup_custom() taking a hash-algorithm parameter.

The old syntax random_setup_special() still exists, and is a macro
wrapper on random_setup_custom() that passes ssh_sha512 as an
argument. This means I can keep the choice of hash function consistent
between the key generation front ends.

This adds potential flexibility: now, anyone wanting a different kind
of special RNG can make it out of whatever primitive they like. But a
more immediate point is to remove an inter-module dependency:
sshrand.c now doesn't need to be linked against the SHA-512 code.

putty.h

@@ -1818,9 +1818,14 @@ void random_unref(void);
  * logical main() no matter whether it needed random numbers or
  * not. */
 void random_clear(void);
-/* random_setup_special is used by PuTTYgen. It makes an extra-big
- * random number generator. */
-void random_setup_special(void);
+/* random_setup_custom sets up the process-global random number
+ * generator specially, with a hash function of your choice. */
+void random_setup_custom(const ssh_hashalg *hash);
+/* random_setup_special() is a macro wrapper on that, which makes an
+ * extra-big one based on SHA-512. It's defined this way to avoid what
+ * would otherwise be an unnecessary module dependency from sshrand.c
+ * to sshsh512.c. */
+#define random_setup_special() random_setup_custom(&ssh_sha512)
 /* Manually drop a random seed into the random number generator, e.g.
  * just before generating a key. */
 void random_reseed(ptrlen seed);

sshrand.c

@@ -19,7 +19,7 @@ int random_active = 0;
  */
 void random_add_noise(NoiseSourceId source, const void *noise, int length) { }
 void random_ref(void) { }
-void random_setup_special(void) { }
+void random_setup_custom(const ssh_hashalg *hash) { }
 void random_unref(void) { }
 void random_read(void *out, size_t size)
 {
@@ -97,10 +97,10 @@ void random_ref(void)
         random_create(&ssh_sha256);
 }
 
-void random_setup_special()
+void random_setup_custom(const ssh_hashalg *hash)
 {
     random_active++;
-    random_create(&ssh_sha512);
+    random_create(hash);
 }
 
 void random_reseed(ptrlen seed)