pam: add a call to pam_namespace

A call to pam_namespace is required so that children of user@.service end up in
a namespace as expected. pam_namespace gets called as part of the stack that
creates a session (login, sshd, gdm, etc.) and those processes end up in a
namespace, but it also needs to be called from our stack which is parallel and
descends from pid1 itself.

The call to pam_namespace is similar to the call to pam_keyinit that was added
in ab79099d16. The pam stack for user@.service
creates a new session which is disconnected from the parent environment. Both
calls are not suitable for inclusion in the shared part of the stack (e.g.
@system-auth on Fedora/RHEL systems), because for example su/sudo/runuser
should not include them.

Fixes #17043 (Allow to execute user service into dedicated namespace
              if pam_namespace enabled)
Related to https://bugzilla.redhat.com/show_bug.cgi?id=1861836
(Polyinstantiation is ignored/bypassed in GNOME sessions)

src/login/systemd-user.in

@@ -15,6 +15,7 @@ session  required pam_selinux.so nottys open
 {% endif %}
 session  required pam_loginuid.so
 session  optional pam_keyinit.so force revoke
+session  required pam_namespace.so
 {% if ENABLE_HOMED %}
 -session  optional pam_systemd_home.so
 {% endif %}