sort: -R no longer disables multithreading

sort: -R no longer disables multithreading

* src/sort.c (random_md5_state): New static var.
(random_md5_state_init): New function, to initialize random_md5_state.
(random_state, randread_source): Remove.
(cmp_hashes): Use random_md5_state rather than random_state.
Break ties using memcmp, not by getting more randomness.
If MD5 collisions turn into a problem in practice, we should
simply use a better checksum.
(main): If -R is given, call random_md5_state_init rather than
going single-threaded.