Make /contrib install/uninstall script consistent:

remove transactions
	use create or replace function
	make formatting consistent
	set search patch on first line

Add documentation on modifying *.sql to set the search patch, and
mention that major upgrades should still run the installation scripts.

Some of these issues were spotted by Tom today.

contrib/btree_gist/uninstall_btree_gist.sql

@@ -1,3 +1,4 @@
+-- Adjust this setting to control where the objects get dropped.
 SET search_path = public;
 
 DROP OPERATOR CLASS gist_cidr_ops USING gist;