Re: pg_rewind in contrib

There are two ways in which access control for replication connections
is separate:

- replication pseudo-database in pg_hba.conf

- replication role attribute

If someone has a restrictive setup for replication and pg_basebackup,
and then pg_rewind enters, they will have to

- add a line to pg_hba.conf to allow non-replication access

- connect as superuser

The first is an inconvenience, although it might be useful for this and
other reasons to have a variant of "all" includes "replication".

The second, however, is a problem, because you have to change from a
restricted, quasi-ready-only user to full superuser access.

One way to address that might be if we added backend functions that are
variants of pg_ls_dir() and pg_stat_file() that are accessible only with
the replication attribute. Or we allow calling pg_ls_dir() and
pg_stat_file() with relative paths with the replication attribute.
(While we're at it, add a recursive variant of pg_ls_dir().) Or some
variant of that.