Patch: Allow formatting in log_line_prefix

Heikki wrote that it might be useful to allow formatting in the
log_line_prefix here
http://www.postgresql.org/message-id/5187CADB.50306@vmware.com

I thought I'd take a bash at implementing space padding part on
log_line_prefix options.

It's been a while since I worked with the PostgreSQL source, so please
forgive me if my coding style is a bit off or the patch is not quite in the
correct format.

It's a little rough around the edges at the moment and likely the
documentation needs more work, but I'm at the stage of wanting to know if
this is even wanted or needed by people.

If you apply this patch you can do things like have %10u %-10d in your
log_line_prefix which will include spaces to give the option the number you
specify as the minimum width. Left and right aligning is supported.

Let this be the thread to collect consensus to whether this needed and
useful.

Regards

David Rowley