Revert to what used to be in shred.c, without

Revert to what used to be in shred.c, without
changing it to allow for varying numbers of words in the state.
Alter so that we include rand-isaac.c directly rather than
compiling it and linking to it.  Don't include config.h or
system.h; that's the includer's responsibility.
(ISAAC_LOG, ISAAC_WORDS, ISAAC_BYTES, struct isaac_state, ind):
(isaac_step, struct irand_state):
Resurrect these, with the same defns that used to be in shred.c.
(ISAAC_SIZE, isaac_new, isaac_copy): Remove.
(isaac_refill, isaac_seed_start, isaac_seed_data, irand_init, irand32):
static again.
(struct isaac_state, isaac_refill, isaac_mix, isaac_init):
(isaac_seed_start, isaac_seed_data, isaac_seed_finish, isaac_seed):
(irand_init, irand32, irand_mod):
Number of words is constant again.