pgbench -i order of vacuum

Is there a reason to vacuum the pgbench_* tables after the indexes on
them are built, rather than before?

Since the indexes are on fresh tables, they can't have anything that
needs to be cleaned.

I don't think the current order accomplishes anything, except to slow
down large initializations by ~25%.

The attached patch moves the vacuums up.

I also made -n skip the vacuums altogether.  Since -n is allowed under
-i, it would be nice if it did something, and there is only one
intuitive thing for it to do.  I don't know what the use case for is,
but I think I've heard grumbling about it before.

Cheers,

Jeff