login: include pam_systemd_home.so in the default PAM snippet we ship for user@.service

Let's make sure systemd-homed works out-of-the box as well as possible.

src/login/systemd-user.m4

@@ -2,11 +2,19 @@
 #
 # Used by systemd --user instances.
 
-account required pam_unix.so
+m4_ifdef(`ENABLE_HOMED',
+-account sufficient pam_systemd_home.so
+)m4_dnl
+account sufficient pam_unix.so
+account required pam_permit.so
+
 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 optional pam_keyinit.so force revoke
+m4_ifdef(`ENABLE_HOMED',
+-session optional pam_systemd_home.so
+)m4_dnl
 session optional pam_systemd.so