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