elogind --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 elogind itself is
concerned, but is nice for various services spawned from by elogind --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