Convert contrib modules to use the extension facility.

This isn't fully tested as yet, in particular I'm not sure that the
"foo--unpackaged--1.0.sql" scripts are OK.  But it's time to get some
buildfarm cycles on it.

sepgsql is not converted to an extension, mainly because it seems to
require a very nonstandard installation process.

Dimitri Fontaine and Tom Lane

contrib/dblink/sql/dblink.sql

@@ -1,17 +1,4 @@
--- Adjust this setting to control where the objects get created.
-SET search_path = public;
-
---
--- Define the functions and test data
--- therein.
---
--- Turn off echoing so that expected file does not depend on
--- contents of dblink.sql.
-SET client_min_messages = warning;
-\set ECHO none
-\i dblink.sql
-\set ECHO all
-RESET client_min_messages;
+CREATE EXTENSION dblink;
 
 CREATE TABLE foo(f1 int, f2 text, f3 text[], primary key (f1,f2));
 INSERT INTO foo VALUES (0,'a','{"a0","b0","c0"}');