maint: use getrandom, not getentropy

maint: use getrandom, not getentropy

This makes for one Gnulib module less, and at runtime there’s
typically just one getrandom syscall instead of several for large
nonces.
* gl/lib/randread.c: Include sys/random.h instead of sys/time.h
and unistd.h.
(get_nonce): Use getrandom, not getentropy.
* gl/modules/randread (Depends-on):
Depend on getrandom, not getentropy.
* src/shred.c (main):
* src/shuf.c (main):
* src/sort.c (random_md5_state_init):
Say "getrandom" rather than "getentropy" in (unlikely) diagnostic.