shuf: improve randperm overflow checking

shuf: improve randperm overflow checking

* gl/lib/randperm.c: Include randperm.h first, since it’s the API.
Include stdint.h, count-leading-zeros.h, verify.h.
(floor_lg): Rename from ceil_log (which was not actually
implementing the ceiling!) and implement the floor using
count_leading_zeros.
(randperm_bound): Use floor_lg, not ceil_log.  Use uintmax_t
instead of size_t in case the size gets large on a 32-bit host.
* gl/modules/randperm (Depends-on): Add count-leading-zeros, stdint.