pgbench - add option to show actual builtin script code

Hello devs,

The minor attached patch $SUBJECT, so that it can be inspected easily,
instead of having to look at the source code or whatever.

sh> pgbench --list select-only
-- select-only: <builtin: select only>
\set aid random(1, 100000 * :scale)
SELECT abalance FROM pgbench_accounts WHERE aid = :aid;

The builtin list output is also slightly improved:

sh> pgbench -b list
Available builtin scripts:
tpcb-like: <builtin: TPC-B (sort of)>
simple-update: <builtin: simple update>
select-only: <builtin: select only>

--
Fabien.