Use new random-number interface rather than rand-isaac.c.

Use new random-number interface rather than rand-isaac.c.
Don't include rand-isaac.c; include randint.h and randread.h instead.
(RANDOM_SOURCE_OPTION): New enum.
(long_opts, usage, main): New option --random-source.
(struct irand_state, irand_init, irand32, irand_mod): Remove.
All callers changed to use randint interface.
(fillrand): Remove.  All callers changed to use randread interface.
(dopass): Remove dependency on ISAAC buffer size.
(genpattern): Don't wipe the random state here.
(randint_source): New static var.
(clear_random_data): New function.
(main): Allocate random source, and arrange to wipe it on exit.