Update user session unit template.

While most folks will be using the derivative from user-session-units,
I'm updating this one to reflect some of the fixes and things to note
about user sessions:

- cgroup should be set with "%u" - username instead of %I
- set dbus path with %U explicitly too
- hint to folks that wish to use MEM_CG features in user sessions
- allow unit to be enabled for instances with systemctl enable

units/user@.service.in

@@ -6,14 +6,18 @@
 #  (at your option) any later version.
 
 [Unit]
-Description=User Manager for %I
+Description=User Manager for %u
 After=systemd-user-sessions.service
 
 [Service]
 User=%I
 PAMName=systemd-shared
-ControlGroup=%R/user/%I/shared cpu:/
+# in order to allow MEM_CG features to work, add "memory:/" here
+ControlGroup=%R/user/%u/shared cpu:/
 ControlGroupModify=yes
 Type=notify
 ExecStart=-@rootlibexecdir@/systemd --user
-Environment=DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/%I/dbus/user_bus_socket
+Environment=DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/%U/dbus/user_bus_socket
+
+[Install]
+Alias=user@%i.service