Throw a useful error message if an extension script file is fed to psql.

We have seen one too many reports of people trying to use 9.1 extension
files in the old-fashioned way of sourcing them in psql.  Not only does
that usually not work (due to failure to substitute for MODULE_PATHNAME
and/or @extschema@), but if it did work they'd get a collection of loose
objects not an extension.  To prevent this, insert an \echo ... \quit
line that prints a suitable error message into each extension script file,
and teach commands/extension.c to ignore lines starting with \echo.
That should not only prevent any adverse consequences of loading a script
file the wrong way, but make it crystal clear to users that they need to
do it differently now.

Tom Lane, following an idea of Andrew Dunstan's.  Back-patch into 9.1
... there is not going to be much value in this if we wait till 9.2.

contrib/btree_gist/btree_gist--1.0.sql

@@ -1,5 +1,8 @@
 /* contrib/btree_gist/btree_gist--1.0.sql */
 
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "CREATE EXTENSION btree_gist" to load this file. \quit
+
 CREATE FUNCTION gbtreekey4_in(cstring)
 RETURNS gbtreekey4
 AS 'MODULE_PATHNAME', 'gbtreekey_in'