patch: SQL/MED(FDW) DDL

Hello.
This is a proposal patch for SQL/MED for 9.1.

At the prev. CF, this patch had so many features to make it hard
to review all of them. So I devided it into smaller parts:
(1) foreign table DDL support (this proposal):
- support for foreign table DDL syntax (CREATE/ALTER FOREIGN TABLE)
- Definition of FDW routine interface and system catalogs for it.
(2) SELECT support for external PostgreSQL tables.

First of all, I'd like to discuss and make agreement on the basic design
and its implementation for FDW interface.
Comments and ideas would be very appriciated, especially
on how system catalog stores the information about
foreign tables, server, and user mappings.

Detail information are described in the wiki:
http://wiki.postgresql.org/wiki/SQL/MED

I'll add this item to the CF site.
And WIP patch for (2) will be available on the page for how (1) is utilized.

- Open Issue:
- fdw connection params issue:
In this implimentation, foreign table connection params(including
passwords) are visible to all users. To avoid it, store them in the
user_mapping object, not in foreign server object.
# It should be improved, though.
- fdw security issue:
It depends on how indivisual fdw extensions are designed.
I'll focus on it after basic agreement on fdw interface.

- prev. CF page:
https://commitfest.postgresql.org/action/patch_view?id=326

- code repository:
http://repo.or.cz/w/pgsql-fdw.git (branch: foreign_table).

repository branches structures is as follows:
master
+foreign_table: (active) branch for this proposal
- foreign_scan: (active) WIP branch for select support
+fdw_ddl: (will be obsolute) branch for last CF
- dblink
- copy_from

Thanks in advance,

SAKAMOTO Masahiko
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center
e-mail: sakamoto(dot)masahiko(at)oss(dot)ntt(dot)co(dot)jp