enable logging of start time/cookie for all backend processes

The attached patch makes a very small but useful change to the behaviour
of log_line_prefix, by enabling the start time (%s) and cookie (%c)
logging to occur for all backends rather than just for session processes
(i.e. backends started for a client connection). We actually need almost
all of this patch, with or without the change in behaviour, so we can
put the cookie in CSVlogs (which I'm still working on), since the
cookie+line number make the natural primary key for the logs. The actual
change in behaviour from this patch comes from the removal of 2 "if
(MyProcPort)" lines in elog.c. Given that, can I sneak this in or should
I wait for 8.4 given we're long past feature freeze?

Most of this has been factored out (and cleaned up) from my previous
CSVlog patch, so it's not new. I did that so that what is left when I
finish will have a rather smaller footprint, as this part seemed fairly
seperable.

cheers

andrew