Specifying the log file name of pgbench -l option

Hi all,

The log file generated by pgbench -l option is fixed file name
'pgbench_log.<pid>.<thread id>'. And it's a little complicated for the
script that runs pgbench repeatedly to identify the log file name.
Attached patch make it possible to specify the log file name. I think
it's useful for the use who want to run pgbench repeatedly in script
and collects and analyze the result.

The one thing I concern is that this patch changes -l option so that
it requires argument.
But changing its behavior would be good rather than adding new option.

Please give me feedback.

Regards,

--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center