pam_systemd: Let user record override env vars

pam_systemd: Let user record override env vars

The user record should be the source of truth for the user's environment
variables, and the user should be able to override them in much the same
way that they can if they simply append the variable to their ~/.profile

For example, before $LANG would never get set to the user's preferred
language, because the service manager always ensures that $LANG is set
to something (either the localed config, or a compiled-in default). Thus
the user's preferredLanguage setting was always ignored