pgsql: Add pg_ls_logdir() and pg_ls_waldir() functions.

Add pg_ls_logdir() and pg_ls_waldir() functions.

These functions are intended to be used by monitoring tools, and,
unlike pg_ls_dir(), access to them can be granted to non-superusers,
so that those monitoring tools can observe the principle of least
privilege.

Dave Page, revised by me, and also reviewed a bit by Thomas Munro.

Discussion: http://postgr.es/m/CA+OCxow-X=D2fWdKy+HP+vQ1LtrgbsYQ=CshzZBqyFT5jOYrFw@mail.gmail.com

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/befd73c50f11a6c6a6719dae20f0de7b7585bef4

Modified Files
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doc/src/sgml/func.sgml | 45 ++++++++++++++++-
src/backend/catalog/system_views.sql | 3 ++
src/backend/utils/adt/genfile.c | 94 ++++++++++++++++++++++++++++++++++++
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.h | 6 +++
5 files changed, 147 insertions(+), 3 deletions(-)