core: imply SetLoginEnvironment= if PAMName= is set

core: imply SetLoginEnvironment= if PAMName= is set

This geneally makes sense as setting up a PAM session pretty much
defines what a login session is.

In context of #30547 this has the benefit that we can take benefit of
the SetLoginEnvironment= effect without having to set it explicitly,
thus retaining some compat of the uid0 client towards older systemd
service managers.