Replace random_byte() with random_read().

This is in preparation for a PRNG revamp which will want to have a
well defined boundary for any given request-for-randomness, so that it
can destroy the evidence afterwards. So no more looping round calling
random_byte() and then stopping when we feel like it: now you say up
front how many random bytes you want, and call random_read() which
gives you that many in one go.

Most of the call sites that had to be fixed are fairly mechanical, and
quite a few ended up more concise afterwards. A few became more
cumbersome, such as mp_random_bits, in which the new API doesn't let
me load the random bytes directly into the target integer without
triggering undefined behaviour, so instead I have to allocate a
separate temporary buffer.

The _most_ interesting call site was in the PKCS#1 v1.5 padding code
in sshrsa.c (used in SSH-1), in which you need a stream of _nonzero_
random bytes. The previous code just looped on random_byte, retrying
if it got a zero. Now I'm doing a much more interesting thing with an
mpint, essentially scaling a binary fraction repeatedly to extract a
number in the range [0,255) and then adding 1 to it.

sshrand.c

@@ -52,9 +52,10 @@ void random_add_noise(void *noise, int length) { }
 void random_add_heavynoise(void *noise, int length) { }
 void random_ref(void) { }
 void random_unref(void) { }
-int random_byte(void)
+void random_read(void *out, size_t size)
 {
     return 0x45; /* Chosen by eight fair coin tosses */
+    memset(out, 0x45, size); /* Chosen by eight fair coin tosses */
 }
 void random_get_savedata(void **data, int *len) { }
 #else /* !FUZZING */
@@ -319,14 +320,17 @@ void random_unref(void)
     random_active--;
 }
 
-int random_byte(void)
+void random_read(void *vout, size_t size)
 {
     assert(random_active);
 
-    if (pool.poolpos >= POOLSIZE)
-	random_stir();
+    uint8_t *out = (uint8_t *)vout;
+    while (size-- > 0) {
+        if (pool.poolpos >= POOLSIZE)
+            random_stir();
 
-    return pool.pool[pool.poolpos++];
+        *out++ = pool.pool[pool.poolpos++];
+    }
 }
 
 void random_get_savedata(void **data, int *len)