For rpm-ostree as used by Atomic Host, we synthesize tmpfiles.d snippets for directories, but not files.

The goal here is that one can perform a "factory reset" by rm -rf /var/* && systemctl reboot.

So currently this requires one to:
touch /var/lib/nfs/{etab,rmtab}

I think the simplest is to change the code to create the file on demand if it doesn't exist.  Failing that, a systemd tmpfiles.d `f` entry would work fine.

It's possible at some point we'll explicitly special case nfs-utils in rpm-ostree to do this, but I'd like to have nfs-utils learn about this as well.