Avoid holding a directory FD open across assorted SRF calls.

This extends the fixes made in commit 085b6b667 to other SRFs with the
same bug, namely pg_logdir_ls(), pgrowlocks(), pg_timezone_names(),
pg_ls_dir(), and pg_tablespace_databases().

Also adjust various comments and documentation to warn against
expecting to clean up resources during a ValuePerCall SRF's final
call.

Back-patch to all supported branches, since these functions were
all born broken.

Justin Pryzby, with cosmetic tweaks by me

Discussion: https://postgr.es/m/20200308173103.GC1357@telsasoft.com

src/test/regress/expected/misc_functions.out

@@ -180,6 +180,27 @@ select count(*) >= 0 as ok from pg_ls_archive_statusdir();
  t
 (1 row)
 
+select * from (select pg_ls_dir('.') a) a where a = 'base' limit 1;
+  a   
+------
+ base
+(1 row)
+
+select * from (select (pg_timezone_names()).name) ptn where name='UTC' limit 1;
+ name 
+------
+ UTC
+(1 row)
+
+select count(*) > 0 from
+  (select pg_tablespace_databases(oid) as pts from pg_tablespace
+   where spcname = 'pg_default') pts
+  join pg_database db on pts.pts = db.oid;
+ ?column? 
+----------
+ t
+(1 row)
+
 --
 -- Test adding a support function to a subject function
 --

src/test/regress/sql/misc_functions.sql

@@ -51,6 +51,15 @@ from (select pg_ls_waldir() w) ss where length((w).name) = 24 limit 1;
 
 select count(*) >= 0 as ok from pg_ls_archive_statusdir();
 
+select * from (select pg_ls_dir('.') a) a where a = 'base' limit 1;
+
+select * from (select (pg_timezone_names()).name) ptn where name='UTC' limit 1;
+
+select count(*) > 0 from
+  (select pg_tablespace_databases(oid) as pts from pg_tablespace
+   where spcname = 'pg_default') pts
+  join pg_database db on pts.pts = db.oid;
+
 --
 -- Test adding a support function to a subject function
 --