systemctl: ignore invalid variables in import-environment
    
    When doing import-environment, we shouldn't fail if some assignment is invalid.
    OTOH, if the invalid assignment is specified as a positional argument, we should
    keep failing.
    
    This would also fix https://bugzilla.redhat.com/show_bug.cgi?id=1754395, by
    ignoring certain variables which are not important in that scenario. It seems
    like the right thing to do in general.