pgsql: Add pg_ls_archive_statusdir function

Add pg_ls_archive_statusdir function

This function lists the contents of the WAL archive status directory,
and is intended to be used by monitoring tools.  Unlike pg_ls_dir(),
access to it can be granted to non-superusers so that those monitoring
tools can observe the principle of least privilege.  Access is also
given by default to members of pg_monitor.

Author:  Christoph Moench-Tegeder
Reviewed-by: Aya Iwata
Discussion: https://postgr.es/m/20180930205920.GA64534@elch.exwg.net

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c48101620151f78ee184e5f81766cbac347d9a11

Modified Files
--------------
doc/src/sgml/func.sgml               | 24 ++++++++++++++++++++++++
src/backend/catalog/system_views.sql |  2 ++
src/backend/utils/adt/genfile.c      |  9 +++++++++
src/include/catalog/catversion.h     |  2 +-
src/include/catalog/pg_proc.dat      |  5 +++++
5 files changed, 41 insertions(+), 1 deletion(-)