core: make sure that if PAMName= is set we always do the full user changing even...

core: make sure that if PAMName= is set we always do the full user changing even if no user is specified explicitly

When PAMName= is set this should be enough to go through our entire user
changing story, so that PAM is definitely run, and environment variables
definitely pulled in and so on.

Previously, it would happen that under some circumstances we might no do
this when transitioning from root to root itself even though PAM was
enabled.

Fixes: #34682