pgbench's arbitrary limit of 10 arguments for SQL statements or
metacommands is far too low. Increase it to 256.

This results in a very modest increase in memory usage, not enough to
worry about.

The maximum includes the SQL statement or metacommand. This is reflected
in the comments and revised TAP tests.

Simon Riggs and Dagfinn Ilmari Mannsåker with some light editing by me.
Reviewed by: David Rowley and Fabien Coelho

Discussion: https://postgr.es/m/CANP8+jJiMJOAf-dLoHuR-8GENiK+eHTY=Omw38Qx7j2g0NDTXA@mail.gmail.com