conf, pasta: With --config-net, copy all routes by default

Use the newly-introduced NL_DUP mode for nl_route() to copy all the
routes associated to the template interface in the outer namespace,
unless --no-copy-routes (also implied by -g) is given.

This option is introduced as deprecated right away: it's not expected
to be of any use, but it's helpful to keep it around for a while to
debug any suspected issue with this change.

Otherwise, we can't use default gateways which are not, address-wise,
on the same subnet as the container, as reported by Callum.

Reported-by: Callum Parsey <callum@neoninteger.au>
Link: https://github.com/containers/podman/issues/18539
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

passt.1

@@ -546,6 +546,21 @@ NAME are given as target), do not exit once the network namespace is deleted.
 Configure networking in the namespace: set up addresses and routes as configured
 or sourced from the host, and bring up the tap interface.
 
+.TP
+.BR \-\-no-copy-routes " " (DEPRECATED)
+With \-\-config-net, do not copy all the routes associated to the interface we
+derive addresses and routes from: set up only the default gateway. Implied by
+-g, \-\-gateway.
+
+Default is to copy all the routing entries from the interface in the outer
+namespace to the target namespace, translating the output interface attribute to
+the outbound interface in the namespace.
+
+Note that this configuration option is \fBdeprecated\fR and will be removed in a
+future version. It is not expected to be of any use, and it simply reflects a
+legacy behaviour. If you have any use for this, refer to \fBREPORTING BUGS\fR
+below.
+
 .TP
 .BR \-\-ns-mac-addr " " \fIaddr
 Configure MAC address \fIaddr\fR on the tap interface in the namespace.