Avoid use of CREATE OR REPLACE FUNCTION in extension installation files.

It was never terribly consistent to use OR REPLACE (because of the lack of
comparable functionality for data types, operators, etc), and
experimentation shows that it's now positively pernicious in the extension
world.  We really want a failure to occur if there are any conflicts, else
it's unclear what the extension-ownership state of the conflicted object
ought to be.  Most of the time, CREATE EXTENSION will fail anyway because
of conflicts on other object types, but an extension defining only
functions can succeed, with bad results.

contrib/lo/lo--1.0.sql

@@ -12,11 +12,11 @@ CREATE DOMAIN lo AS pg_catalog.oid;
 -- The other functions that formerly existed are not needed because
 -- the implicit casts between a domain and its underlying type handle them.
 --
-CREATE OR REPLACE FUNCTION lo_oid(lo) RETURNS pg_catalog.oid AS
+CREATE FUNCTION lo_oid(lo) RETURNS pg_catalog.oid AS
 'SELECT $1::pg_catalog.oid' LANGUAGE SQL STRICT IMMUTABLE;
 
 -- This is used in triggers
-CREATE OR REPLACE FUNCTION lo_manage()
+CREATE FUNCTION lo_manage()
 RETURNS pg_catalog.trigger
 AS 'MODULE_PATHNAME'
 LANGUAGE C;