Add regression test to prevent future breakage of legacy query in libpq.

Memorialize the expected output of the query that libpq has been using for
many years to get the OIDs of large-object support functions.  Although
we really ought to change the way libpq does this, we must expect that
this query will remain in use in the field for the foreseeable future,
so until we're ready to break compatibility with old libpq versions
we'd better check the results stay the same.  See the recent lo_create()
fiasco.

src/test/regress/expected/opr_sanity.out

@@ -638,6 +638,46 @@ uuid_gt(uuid,uuid)
 uuid_ne(uuid,uuid)
 -- restore normal output mode
 \a\t
+-- List of functions used by libpq's fe-lobj.c
+--
+-- If the output of this query changes, you probably broke libpq.
+-- lo_initialize() assumes that there will be at most one match for
+-- each listed name.
+select proname, oid from pg_catalog.pg_proc
+where proname in (
+  'lo_open',
+  'lo_close',
+  'lo_creat',
+  'lo_create',
+  'lo_unlink',
+  'lo_lseek',
+  'lo_lseek64',
+  'lo_tell',
+  'lo_tell64',
+  'lo_truncate',
+  'lo_truncate64',
+  'loread',
+  'lowrite')
+and pronamespace = (select oid from pg_catalog.pg_namespace
+                    where nspname = 'pg_catalog')
+order by 1;
+    proname    | oid  
+---------------+------
+ lo_close      |  953
+ lo_creat      |  957
+ lo_create     |  715
+ lo_lseek      |  956
+ lo_lseek64    | 3170
+ lo_open       |  952
+ lo_tell       |  958
+ lo_tell64     | 3171
+ lo_truncate   | 1004
+ lo_truncate64 | 3172
+ lo_unlink     |  964
+ loread        |  954
+ lowrite       |  955
+(13 rows)
+
 -- **************** pg_cast ****************
 -- Catch bogus values in pg_cast columns (other than cases detected by
 -- oidjoins test).

src/test/regress/sql/opr_sanity.sql

@@ -317,6 +317,30 @@ ORDER BY 1;
 -- restore normal output mode
 \a\t
 
+-- List of functions used by libpq's fe-lobj.c
+--
+-- If the output of this query changes, you probably broke libpq.
+-- lo_initialize() assumes that there will be at most one match for
+-- each listed name.
+select proname, oid from pg_catalog.pg_proc
+where proname in (
+  'lo_open',
+  'lo_close',
+  'lo_creat',
+  'lo_create',
+  'lo_unlink',
+  'lo_lseek',
+  'lo_lseek64',
+  'lo_tell',
+  'lo_tell64',
+  'lo_truncate',
+  'lo_truncate64',
+  'loread',
+  'lowrite')
+and pronamespace = (select oid from pg_catalog.pg_namespace
+                    where nspname = 'pg_catalog')
+order by 1;
+
 
 -- **************** pg_cast ****************