pgbench: allow a script weight of zero

pgbench: allow a script weight of zero

This refines the previous weight range and allows a script to be "turned
off" by passing a zero weight, which is useful when scripting multiple
pgbench runs.

I did not apply the suggested warning when a script uses zero weight; we
use the principle elsewhere that if there's nothing to be done, do
nothing quietly.

Adjust docs accordingly.

Author: Jeff Janes, Fabien Coelho