per-column FDW options, v5

Here is a rebased version of per-column FDW options patch. I've
proposed this patch in last CF, but it was marked as returned with
feedback. So I would like to propose in next CF 2011-09. I already
moved CF item into new topic "SQL/MED" of CF 2011-09.

I didn't change the patch except rebasing against head, so this patch
includes changes below:

* New attribute attfdwoptions of pg_attribute hold per-column FDW
options, so this patch needs bumping catversion.
* CREATE/ALTER FOREIGN TABLE statement accept OPTIONS (key 'value')
syntax for per-column FDW options.
* New information_schema.column_options view shows per-column FDW
options of accessible foreign tables.
* \d+ shows per-column options, only for foreign tables.
* pg_dump support dumping per-column FDW options as ALTER FOREIGN TABLE
statement.

IIUC, one of the reasons why this patch hasn't been accepted is that we
don't have a consensus about storage for per-column FDW options. I
think that separating attributes would be better for several reasons:

* FDW options might conflict reloptions/attoptions.
* FDW options and reloptions have different syntax, so pg_dump would
need to switch the format depending on target table's relkind.

I also attached a rebased version of force_not_null patch, which adds
force_not_null option support to file_fdw. This is a use case of
per-column FDW option.

Regards,
--
Shigeru Hanada

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