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.
Include md5.h, randread.h, xmemxfrm.h.
(longopts, usage, main): Remove undocumented --seed option;
it's now replaced by --random-source.
(rand_state, get_hash): Remove.
(randread_source): New static var.
(random_state, cmp_hashes, compare_random): New functions; they guarantee
no collisions in the random hash function.
(keycompare): Use compare_random for -R; don't fall back on comparing
via memcoll, since compare_random does the right thing.