sysinit.target: Don't depend on systemd-tmpfiles-setup.service

systemd-tmpfiles-setup.service pulls in local-fs.target, which
interferes with NixOps' send-keys feature (since sshd.service depends
indirectly on sysinit.target). Since in NixOS we don't use
systemd-tmpfiles for creating files (that's done by activation scripts
and preStart scripts), it's not a problem to start it a bit later.
systemd-journal-flush: Require /var/log/journal rather than all filesystems