SQL/MED - core functionality

Hi hackers,

Attached is a patch that adds core functionality of SQL/MED. This
patch provides:

* new option HANDLER for FOREIGN DATA WRAPPER
* CREATE/ALTER DDLs are supported
* psql \dew command shows handler option too
* pg_dump can dump HANDLER option

* new object type FOREIGN TABLE
* CREATE/ALTER/DROP DDLs are supported
* system columns except TABLEOID are not supported
* inheriting normal table is supported
* psql \d shows detail of foreign tables
* psql \det lists foreign tables
* psql \dE lists foreign tables in \d format
* pg_dump can dump the definition
* information_schema views added
* foreign table is read-only, so INSERT/UPDATE/DELETE are denied
* ANALYZE and VACUUM skips foreign tables

* new executor node ForeignScan
* it's a counterpart of SeqScan
* this node scans one foreign table at a time
* FDW HANDLER is necessary to execute SELECT statement

Patches for FDWs which can be used to execute SELECT statement will be
posted in their own thread soon.

"SQL/MED - file_fdw" : FDW for external PostgreSQL
"SEL/MED - postgresql_fdw" : FDW for server-side file (CSV, TEXT)

I would reuse existing CommitFest item "SQL/MED" for this patch, and
add new item for each FDW patch.

Regards,
--
Shigeru Hanada