pgsql: Avoid holding a directory FD open across assorted SRF calls.

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b4570d33aa045df330bb325ba8a2cbf02266a555

Modified Files
--------------
contrib/adminpack/adminpack.c | 78 ++++++------
contrib/pgrowlocks/pgrowlocks.c | 173 ++++++++++++---------------
doc/src/sgml/xfunc.sgml | 96 +++++++++------
src/backend/utils/adt/datetime.c | 101 +++++++---------
src/backend/utils/adt/genfile.c | 112 ++++++++---------
src/backend/utils/adt/misc.c | 117 ++++++++++--------
src/backend/utils/fmgr/README | 16 ++-
src/include/funcapi.h | 13 +-
src/test/regress/expected/misc_functions.out | 21 ++++
src/test/regress/sql/misc_functions.sql | 9 ++
10 files changed, 388 insertions(+), 348 deletions(-)