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
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/819d5525744bff600b6073804a7a5eb7a9342a8b

Modified Files
--------------
contrib/adminpack/adminpack.c | 80 +++++++------
contrib/pgrowlocks/pgrowlocks.c | 164 ++++++++++++---------------
doc/src/sgml/xfunc.sgml | 96 ++++++++++------
src/backend/utils/adt/datetime.c | 101 +++++++----------
src/backend/utils/adt/genfile.c | 109 +++++++++---------
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, 385 insertions(+), 341 deletions(-)