Windows PuTTYgen: bound entropy input by PRNG state size.

Although I've reinstated the tedious manual mouse input, I can at
least reduce the amount of it that the user is required to provide:
the new PRNG has a hard limit on the size of its seed, so once we've
generated enough entropy to fill that up, there's no point in
collecting more, even if we're generating a particularly large key.

sshprng.c

@@ -284,3 +284,9 @@ void prng_add_entropy(prng *pr, unsigned source_id, ptrlen data)
         prng_seed_finish(&pi->Prng);
     }
 }
+
+size_t prng_seed_bits(prng *pr)
+{
+    prng_impl *pi = container_of(pr, prng_impl, Prng);
+    return pi->hashalg->hlen * 8;
+}