pg_settings view

As discussed on several occasions previously, the new anonymous
composite type capability makes it possible to create a system view
based on a table function in a way that is hopefully palatable to
everyone. The attached patch takes advantage of this, moving
show_all_settings() from contrib/tablefunc into the backend (renamed
all_settings(). It is defined as a builtin returning type RECORD. During
initdb a system view is created to expose the same information presently
available through SHOW ALL. For example:

test=# select * from pg_settings where name like '%debug%';
name | setting
-----------------------+---------
debug_assertions | on
debug_pretty_print | off
debug_print_parse | off
debug_print_plan | off
debug_print_query | off
debug_print_rewritten | off
wal_debug | 0
(7 rows)

Additionally during initdb two rules are created which make it possible
to change settings by updating the system view -- a "virtual table" as
Tom put it. Here's an example:

test=# update pg_settings set setting = 'on' where name =
'show_query_stats';
set_config
------------
on
(1 row)

test=# update pg_settings set setting = 'off' where name =
'show_query_stats';
set_config
------------
off
(1 row)

test=# update pg_settings set setting = 'off' where name = 'does not exist';
set_config
------------
(0 rows)

test=# update pg_settings set name = 'why would I do this?' where name =
'show_query_stats';
set_config
------------
(0 rows)

I have adjusted the regression test for rules also. All regression tests
passed.

I have not revised the docs yet. I wanted to see if this would be
accepted before spending time on the documentation, because it has gone
back and forth more than once already ;-)

If there are no objections, please apply.

Thanks,

Joe