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.

pageant.c

@@ -21,11 +21,9 @@
  * won't generate true random numbers. So we must scream, panic,
  * and exit immediately if that should happen.
  */
-int random_byte(void)
+void random_read(void *buf, size_t size)
 {
     modalfatalbox("Internal error: attempt to use random numbers in Pageant");
-    exit(0);
-    return 0;                 /* unreachable, but placate optimiser */
 }
 
 static bool pageant_local = false;