systemd --user: call pam_loginuid when creating user@.service (#3120)

This way the user service will have a loginuid, and it will be inherited by
child services. This shouldn't change anything as far as systemd itself is
concerned, but is nice for various services spawned from by systemd --user
that expect a loginuid.

pam_loginuid(8) says that it should be enabled for "..., crond and atd".
user@.service should behave similarly to those two as far as audit is
concerned.

https://bugzilla.redhat.com/show_bug.cgi?id=1328947#c28

src/login/systemd-user.m4

@@ -8,4 +8,5 @@ m4_ifdef(`HAVE_SELINUX',
 session  required pam_selinux.so close
 session  required pam_selinux.so nottys open
 )m4_dnl
+session  required pam_loginuid.so
 session  include system-auth