I was confused a bit by the wrong construction with RAND_MAX

I was confused a bit by the wrong construction with RAND_MAX
used in randomize_fd.c.
Although the max value is the same currently, RAND_MAX is for rand(),
not for random().
So move RANDOM_MAX const to the common file now, make it UL and
use in randomize_fd.c too.
(in any case its old value was 1 less then must be, as noted in
the prev. commit)