Listen on both loopback addresses, and name them

[marcos-mendez]

The layer shipped a database an appliance built on it could not reach
over IPv6, which is the address family this project builds for.

conf.d/main asserted Debian's default, listen_addresses = 'localhost',
on the grounds that it is the loopback of both families. It is not.
Debian's /etc/hosts maps ::1 to ip6-localhost and ip6-loopback and never
to localhost, so getaddrinfo("localhost") answers 127.0.0.1 alone and the
cluster binds the IPv4 loopback only. Measured on a container booted from
the published layer:

# ss -lntH 'sport = :5432'
LISTEN 0  200  127.0.0.1:5432  0.0.0.0:*

with ::1 up on lo and pg_hba.conf already carrying its scram-sha-256 line
for ::1/128, ready for a connection that could never arrive. That is what
failed firstboot.d/36pgsqlverify on the gate, and the hook was right:
35pgsqlpass had set the password over the unix socket and the database
was still unreachable at the address this layer says it serves.

The fix is two literal addresses, '::1,127.0.0.1', the same pair the
MariaDB layer writes into its bind-address. A literal address cannot
resolve into something else, which is the point: the previous line was
not wrong about what it wanted, it was wrong about what a name meant on
this machine. What it binds is now proved on the booted machine by the
boot test rather than assumed by a grep of the setting here, which is the
lesson of the defect: asserting the configuration is not asserting the
behaviour.

Measured on the build host, on a container booted from the published
chain with the line in place:

# ss -lntH 'sport = :5432'
LISTEN 0  200  127.0.0.1:5432  0.0.0.0:*
LISTEN 0  200      [::1]:5432     [::]:*

$ psql --username=postgres --host=::1 --port=5432 --no-password \
    --command='SELECT 1, current_user, inet_server_addr()'
1|postgres|::1

with the declared password in PGPASSWORD, and with the wrong one

FATAL:  password authentication failed for user "postgres"

The layer ships this, so the changelog gains an entry and the layer needs
a rebuild before its gate can go green.