csvlog_fields review

It bit rotted a bit find a new version attached that includes the
following fixes:

- show_session_authorization() no longer exists, instead access the
session_authorization_guc directly (like we do for show_role in
commands/variable.c). I find it quite ugly tho...
- it changed %u to %U and %U to be %u... flipped it back so %u remains unchanged
- num_fields in write_csvlog was unused, removed it
- new_csv_fields would leak in an error path of assign_csvlog_fields
(which probably matters as we are in TopMemoryContext)

All of Itagaki-san's comments still stand:

> * csvlog_fields and csvlog_header won't work with non-default log_filename
when it doesn't contain seconds in the format. They expect they can always
open empty log files.

I think at the very least we should document this? Or maybe only write
out the header when its a zero length file?

> * The long default value for csvlog_fields leads long text line in
postgresql.conf, SHOW ALL, pg_settings view, but there were no better
alternative solutions in the past discussion.

I think it might be worth revisiting using the %X syntax
log_line_prefix uses instead of inventing our own long form names.

> * csvlog_fields is marked as PGC_POSTMASTER. It can protect mixed formats
in a csv file on default log_filename, but other similar GUC variables
are usually marked AS PGC_SIGHUP.

I don't really see this as a problem...

As it stands I think we still need to address the first two questions
before its ready for a -commiter.