tmpfiles: there's no systemd-forbid-user-logins.service service

tmpfiles: introduce the concept of unsafe operations Various operations done by systemd-tmpfiles may only be safely done at boot (e.g. removal of X lockfiles in /tmp, creation of /run/nologin). Other operations may be done at any point in time (e.g. setting the ownership on /{run,var}/log/journal). This distinction is largely orthogonal to the type of operation. A new switch --unsafe is added, and operations which should only be executed during bootup are marked with an exclamation mark in the configuration files. systemd-tmpfiles.service is modified to use this switch, and guards are added so it is hard to re-start it by mistake. If we install a new version of systemd, we actually want to enforce some changes to tmpfiles configuration immediately. This should now be possible to do safely, so distribution packages can be modified to execute the "safe" subset at package installation time. /run/nologin creation is split out into a separate service, to make it easy to override. https://bugzilla.redhat.com/show_bug.cgi?id=1043212 https://bugzilla.redhat.com/show_bug.cgi?id=1045849