Add default_val to pg_settings

Attached is an updated and separate version of my patch exposing the
internal GUC boot_val as default_val, which failed to attach itself to the
already committed changes to the pg_settings view.

Brief reminder of what it does:

postgres=# select name,setting,default_val from pg_settings where name='shared_buffers';
name | setting | default_val
----------------+---------+-------------
shared_buffers | 4096 | 1024

General justification for this change with a longer example is at
http://archives.postgresql.org/pgsql-hackers/2008-09/msg00233.php

Based on feedback the first time around, I updated the documentation for
this column to read "Default value assumed at server startup if the
parameter is not otherwise set".

Would only take a quick search/replace of the patch to change
"default_val" to something else if there are still any objections there.
"boot_val" is another candidate name, I feel that would make the purpose
of the column less obvious to users of pg_settings even if it is more
correct. I'm really more concerned about the feature than exactly what
it's named though. I didn't bother to expose the reset_val since I can't
think of any obvious use cases for wanting to know it.

--
* Greg Smith gsmith(at)gregsmith(dot)com http://www.gregsmith.com Baltimore, MD