random-util: rename acquire_random_bytes() → genuine_random_bytes()

random-util: rename acquire_random_bytes() → genuine_random_bytes()

It's more descriptive, since we also have a function random_bytes()
which sounds very similar.

Also rename pseudorandom_bytes() to pseudo_random_bytes(). This way the
two functions are nicely systematic, one returning genuine random bytes
and the other pseudo random ones.