Avoiding inadvertent debugging mode for pgbench

Attached please find a patch to adjust the behavior of the pgbench program
and make it behave like the other programs that connect to a database
(namely, psql and pg_dump). Specifically, add support for using -d and
--dbname to specify the name of the database. This means that -d can no
longer be used to turn on debugging mode, and the long option --debug must
be used instead.

This removes a long-standing footgun, in which people assume that the -d
option behaves the same as other programs. Indeed, because it takes no
arguments, and because the first non-option argument is the database name,
it still appears to work. However, people then wonder why pgbench is so
darn verbose all the time! :)

This is a breaking change, but fixing it this way seems to have the least
total impact, as the number of people using the debug mode of pgbench is
likely quite small. Further, those already using the long option are
unaffected, and those using the short one simply need to replace '-d' with
'--debug', arguably making their scripts a little more self-documenting in
the process.

Cheers,
Greg