pgbench: Make set_random_seed() 64-bit everywhere.

pgbench: Make set_random_seed() 64-bit everywhere.

Delete an intermediate variable, a redundant cast, a use of long and a
use of long long.  scanf() the seed directly into a uint64, now that we
can do that with SCNu64 from <inttypes.h>.

The previous coding was from pre-C99 times when %lld might not have been
there, so it read into an unsigned long.  Therefore behavior varied
by OS, and --random-seed would accept either 32 or 64 bit seeds.  Now
it's the same everywhere.

Author: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/b936d2fb-590d-49c3-a615-92c3a88c6c19%40eisentraut.org