Also remove some unused code and the no longer useful dblink.h file.

Reviewed-by: Tsunakawa, Takayuki <tsunakawa.takay@jp.fujitsu.com>

Prototypes for functions implementing V1-callable functions are no
longer necessary.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Thomas Munro <thomas.munro@enterprisedb.com>

Backpatch certain files through 9.1

Backpatch certain files through 9.0

Update all files in head, and files COPYRIGHT and legal.sgml in all back
branches.

Fully update git head, and update back branches in ./COPYRIGHT and
legal.sgml files.

dblink now has its own validator function dblink_fdw_validator(), which is
better than the core function postgresql_fdw_validator() because it gets
the list of legal options from libpq instead of having a hard-wired list.

Make the dblink extension module provide a standard foreign data wrapper
dblink_fdw that encapsulates use of this validator, and recommend use of
that wrapper instead of making up wrappers on the fly.

Unfortunately, because ad-hoc wrappers *were* recommended practice
previously, it's not clear when we can get rid of postgresql_fdw_validator
without causing upgrade problems.  But this is a step in the right
direction.

Shigeru Hanada, reviewed by KaiGai Kohei