Enable replication connections by default in pg_hba.conf

initdb now initializes a pg_hba.conf that allows replication connections
from the local host, same as it does for regular connections.  The
connecting user still needs to have the REPLICATION attribute or be a
superuser.

The intent is to allow pg_basebackup from the local host to succeed
without requiring additional configuration.

Michael Paquier <michael.paquier@gmail.com> and me