pg_ls_waldir() & pg_ls_logdir()

Hi

Following various conversations on list and in person, including the
developer meeting in Brussels earlier this month, here is a patch that
implements pg_ls_logdir() and pg_ls_waldir() functions.

The ultimate aim of this (and followup work I'll be doing) is to
provide functionality to enable monitoring of PostgreSQL without
requiring a user with superuser permissions as many of us have users
for whom security policies prevent this or make it very difficult.

In order to achieve that, there are various pieces of functionality
such as pg_ls_dir() that need to have superuser checks removed to
allow permissions to be granted to a monitoring role. There were
objections in previous discussions to doing this with such generic
functions, hence this patch which adds two narrowly focussed functions
to allow tools to monitor the contents of the log and WAL directories.
Neither function has a hard-coded superuser check, but have ACLs that
prevent public execution by default.

Patch includes the code and doc updates.

--
Dave Page
Blog: http://pgsnake.blogspot.com
Twitter: @pgsnake

EnterpriseDB UK: http://www.enterprisedb.com
The Enterprise PostgreSQL Company